Most effective way to improve cantilever brakes

The best option would probably be to change to (Mini) V-Brakes, at least for the front. For touring you probably also want to change the rear brake. If I’m not mistaken the Shimano Sora brake levers are of the short pull type and need Mini V-Brakes (e.g. XLC BR-V04 or TRP CX8.4).

Their only disadvantage compared to Cantilevers is that they have less clearance (for mud, wide tires, mudguards) and the pads sit closer to the rim.

In my experience Cantilever brakes are extremely bad, even in dry conditions. Getting the best brake pads, best cables, best (Cantilever) brakes, rims with special plating etc. will slightly help, but it will never make them good or even great.

See also: How do Mini V-brakes compare to cantilever brakes for Cyclocross bikes?

Cantis are good brakes and can work really well. Their downside is a little more fiddliness to set up.

Some things to check:

• Brake pads - if they are over 5 years old, just replace them. Brake pads/blocks get hard with age and don't function as well. If they're pretty new, clean out any embedded metal with a sharp pick. Can do this on the bike if you undo the straddle cable
• Rim cleaning - improve the surface of your rims by cleaning with some kind of spirits (degreasers may leave a film), then give them a very light sand to add some tooth. If you feel high spots in the metal, smooth them off.
• True your wheels - wobbly wheels means brake pads have to rest further apart which means there's slack to take-up before they bite.
• Pad alignment - aim to have the pads aligned with the side of the rim and not overlapping the tyre or the corner. Also aim to toe them in properly
• The straddle cable should be positioned properly - the cable should leave the brake arms at a 90 degree angle, and the hanger/hook on the main cable should be a nice curve. The height of this hook can vary, I can't recommend a good angle here.
• Do you have steel or chromed rims? If so, look to replacing them with aluminium rims. Do the front first if you can only do one wheel.

I felt more confidence in my canti brakes when the safety catcher was installed under the straddle cable. A good mudguard does the same job, of catching the straddle cable should the main brake cable part. If this happened, the straddle cable could lock up the wheel very quickly.

Cantis make perfectly adequate brakes, they became less popular when V brakes took less adjustment overall, and cantis can be vulnerable because they stick out sideways a lot more, also being less aero. But for a touring bike there's probably a front rack in the way already.

You can doubtless improve your brakes, just take your time and don't be pressured to have it fixed ready for the next day.

I have a Dawes Super Galaxy touring bike with cantilever brakes, and I'm a bit worried about the stopping power of the brakes when I start loading the bike up with kit to go touring.

Don't be worried.

Unlike single pivot sidepull calipers that suffer from uneven centering, or dual pivot sidepulls that cannot track a wobbly wheel due to forced centering, cantilever brakes have a perfectly working centering mechanism and are able to track a wobbly wheel. Thus, unlike single pivot sidepull calipers that have a low mechanical advantage due to their flaky centering, cantilevers can have a high mechanical advantage.

Not only that, but cantilever brakes are unique among brakes that their mechanical advantage is adjustable. Single pivot sidepulls, dual pivot sidepulls, and V brakes all have a fixed mechanical advantage. To get a different mechanical advantage, you have to replace the entire set of brakes or brake levers. With cantilevers, you can adjust the mechanical advantage in two different ways: by adjusting pad attachment and by adjusting the straddle cable or exchanging the link wire.

About the only five problems of cantilever brakes are:

• The pad moves down as it wears so pads cannot be very thick (applies to V brakes as well) -- fortunately, nowadays manufacturers make pads for cantilevers that are long, thin and curved to match the wheel curvature. So with proper pads you never have to change the pad position as it wears.
• The mechanical advantage goes lower as the pad approaches the rim and even lower as the pad wears. However, this is not a major problem -- the benefit of the ability to adjust the mechanical advantage is a far more important feature than the slight reduction in mechanical advantage as the pad approaches the rim, and besides, as I already said cantilevers should use thin pads so the lowering of mechanical advantage as the pad wears is not a problem either.
• In wet conditions, the water on the rim takes about two wheel revolutions to dissipate so if braking suddenly you move bit over 4 meters until you have full braking force. This is not an issue at all for an experienced cyclist: on any bike a cyclist anticipating the need to stop lightly presses on the brake lever to take up the slack on the braking system. On a rim brake, you just slightly increase the braking force to be enough to dissipate the water in advance, so when you decide that you need to stop, the rims are already dry.
• Some setups (knobby tyre, no fender, no cable catcher, straddle cable) have a hazard in which if the main cable breaks, the straddle cable falls on the knobby tyre because there's no fender or cable catcher, and locks the brake, causing you to fly over the bars. The fix is to install a fender or a cable cather such as a front reflector or headlight or a specific cable catcher, or to change to a link wire where half of it is the main cable and thus it can't lock up the brake on main cable breakage.
• Cantilever brakes require a cable stop on the frame or fork, which makes their use difficult on suspension bicycles -- but of course for non-suspension bicycles this is not an issue

My touring bike (Surly Long Haul Trucker) has Shimano BR-R550 cantilevers and BL-R400 levers, with brakes using link wire "E" and about 1.5x mechanical advantage -- about the same as dual pivot sidepulls. Despite the fact that I built the front wheel on a low precision truing stand (so it's not very true), the brake pads never rub on the rim unless I brake -- compare that to disc brakes where the brake pads routinely rub on the rotor. The brake is equally powerful to Shimano BR-4770 hydraulic disc brakes on my e-bike, but wins on firmness -- when braking hard on the BR-4770 brakes, the brake lever feels spongy but not so on the cantilevers. Also the mechanical cantilevers win on simplicity.

Today everyone is happily using hydraulic disc brakes on even road bikes, despite the fact that a far better alternative exists: the cantilever brake, with its adjustable mechanical advantage, simple maintenance, no pad rub, very firm braking feel and as good braking force for equal lever force.

But I don't know what the most effective method to improve the braking power would be. I've listed below some options I would consider, but I've also included what I don't know which could affect each measure.

• Change the brake blocks. Perhaps more expensive brake blocks are much more effective.

All quality brake blocks withstand even sunlight (I stored a touring bike one year in sunlight -- the rubber of the brake lever hoods degraded but the brake pads brake just fine) and brake well. No need to change to other brake blocks for those reasons. However, there is a reason to prefer Kool Stop salmon colored brake blocks (NOT the dual compounds that are 50% salmon and 50% crap), because the salmon blocks don't accumulate grit that wears rims quickly. So rim life is a reason to prefer quality brake blocks, but braking force is not a reason.

• Check your brake adjustment. Perhaps there are common and simple mistakes people make when setting up cantilever brakes.

Indeed, you nailed it. There are two adjustments:

• Straddle cable length or link wire length (straddle cable length is adjustable, link wire length is changed by changing to another form of link wire)
• Pad spacers on the mounting hardware

To get as high mechanical advantage as possible, you want to:

• Put all pad spacers between the pad and the brake arm, so that as little spacers as possible are between the brake arm and the tightening nut (NOTE: you need to release cable tension somewhat by unbolting the cable anchor, loosening the cable and tightening the cable anchor so you get space for more pad spacers, NOTE2: some cantilevers use pads with a post; they don't have pad spacers but you can still adjust the location where the post is on the brake arm pad mounting hardware)
• Use as short straddle cable / link wire as possible so the angle between the two segments on the sides is as close to 180 degrees as you can make it

For example, my BR-R550 brakes are supplied with link wire E (79.5 mm) and F (84.5 mm). Of these, the E gives about 10% more mechanical advantage if using the same pad spacer arrangement. By arranging the pad spacers differently, you can get more difference in the mechanical advantage.

• Change to newer cantilever brakes. Perhaps the old style cannot be fixed and it's best to go for more modern cantilvers.

No. Nearly every cantilever brake has enough options to adjust the mechanical advantage to suit to your tastes. About the only thing you might need to do is to purchase a different length of link wire.

• Change to a different type of brake. Perhaps cantilevers as a whole just aren't very good and my forks are compatible with a different brake style.

No. Cantilevers are unique among bicycle brakes because their mechanical advantage can be adjusted. By purchasing some other brake type, you are throwing away your adjustment opportunities. Learn how to adjust cantilever brakes, instead. If you want high mechanical advantage, you can have it with cantilevers.

• Buy new rims. Perhaps my rims are made of a notoriously slippery material and changing anything about the brakes will never have that large an effect.

Only if you have steel rims that take forever to dissipate water in wet conditions and have practically no braking until the water is dissipated in contrast to aluminum rims that have some braking even before the water is dissipated and take only two wheel revolutions to dissipate the water.

However, there are today rims that are not intended for rim brakes at all, disc specific rims. Those don't have parallel brake tracks and shouldn't be used. Also rims that are made from carbon fiber or wood, or rims that have aluminum anodizing on the brake tracks should be avoided.

I'd be grateful if people could point me in the direction of finding out which is the single biggest change I can make for better braking performance.

You probably haven't fine tuned your mechanical advantage by adjusting the pad spacers and the straddle cable / link wire length. By those adjustment possibilities, you can adjust the mechanical advantage for free (pad spacers, straddle cable length) or for very minimal money (a different length link wire costs less than 5 euros per brake).

Also a final note: brake levers intended for V brakes cannot be used for cantilever brakes. You'll have less than half of the intended braking force if using V brake specific levers.