Prepping pedals when installing

Pedals are not intrinsically self-tightening. If they are tightened sufficiently in the first place they will not get tighter by themselves, at least not in the sense of rotating further into the cranks. You can confirm this by marking the pedals and cranks and monitoring them over time.

The fact that the left pedals have left hand threads is largely historical. A properly tightened pedal will not come loose even if the left hand pedal is RH threaded. This is proven by tandem riders who use RH cranks on the left side. There is still a small benefit to LH threaded pedals in certain circumstances, such as if the pedals are already too loose, so overall there is a small benefit of the LH threads, but the thread direction by itself is not contributing to your problem as long as you put the correct pedal on the correct side.

If pedals are difficult to remove it usually means they suffered from corrosion or they were simply overtightened. In rare cases the crank arms may have been damaged by impact or bending that distorts the pedal threads. Pedal threads can still corrode even when greased, especially if they are threaded into aluminum. Basic grease often contains enough water or other conductive ingredients to allow electrogalvanic corrosion to take place. To avoid this further you can use specific anti-sieze compound or at least dielectric grease.

When installing pedals it's recommended to use grease or anti-sieze. Tighten sufficiently but not excessively. A standard pedal wrench like a Park tool pedal wrench should be the right length so that the tightness will be about right if you tighten them about as tight as you can get them. Do not use a cheater bar or wrap a rag around the pedal wrench when installing, but you can try either of those things when removing pedals.

It's not usually recommended to remove and reinstall pedals just to avoid seizing. But if you are having specific problems then removing, cleaning, re-greasing and properly reinstalling would be a reasonable thing to do, perhaps once a year or something.

Sounds like you tightened them too tight.

You did everything else right - pedals on the right side, threaded the right way, grease.

So, if you had to use a 6 foot cheater bar you might have used a little too much muscle when tightening.

According to bikeride.com pedals should be torqued to something between 276 and 354 inch pounds or 29 to 40 Newton Meters depending on the crank arm.

Campgnolo = 40 Nm or 354 in lbs
FSA = 29 to 34 NM or 257 to 301 in lbs
Ritchey = 35 Nm or 307 in lbs
Shimano = 35 Nm or 304 in lbs

There are a number of issues in the pedal-crank joint:

1. They fail. Early and often. If a heavyweight rider uses the same crank for a period of years to tens of years and rides a lot, it soon becomes apparent that the crank is indeed weaker than for example the chainwheel. If will fail due to cracking at the pedal eye.
2. They sometimes become so tight that untightening is a Herculean effort. Grease helps but doesn't completely remove all problems.
3. The pedal eye of a used crank shows wear in the crank. This is evidence of the pedal moving in use in the joint and slowly wearing away the crank.
4. There is a left-hand thread in the left pedal. This is too evidence that something has been misdesigned. There is no excuse for left-hand threads. If a left-hand thread is needed to keep something tight, the entire joint has been misdesigned.

For details, see http://pardo.net/bike/pic/mobi/d.pedal-crank-joint/index.html

The best you can do to avoid the issue with standard joint is to always grease the pedals when installing and tightening both pedals to proper tightness. The pedal moves in the joint with use but this cannot be avoided with the standard joint.

Stuck pedals can be removed with heat.

If you want to prevent early crank failures and solve the issue of pedals sometimes screwing themselves really tight, see the split collet solution in http://pardo.net/bike/pic/mobi/d.pedal-crank-joint/index.html -- it is not easy to make the solution but with access to machine shop, it is possible and solves all of the issues in the joint except the left-hand thread (but the left-hand thread causes no real harm).

I would just add that a clean thread is a happy thread. Whenever threads are stiff or sticking, a good clean will usually improve things considerably. In this case the best tool for the job is an old toothbrush with the bristles shortened to about half their original length, or a little more than that (it's a bit fiddly but it's worth it). Use some kind of solvent if necessary to get the male and female threads really clean. When they have dried, apply a little copper grease or anti-seize and reassemble. Torque-wise, it could be difficult to get a torque wrench on there, but it's not really necessary. I use a standard length combination wrench and lean on it reasonably firmly. It needs to be good and tight but not super tight (nothing like a lug nut on a car, for example) When removing a pedal, I position the crank so that I can put my foot on the wrench and loosen it just by pushing down with my foot. It avoids any chance of a skinned knuckle and it's just more comfortable when the pedal has been tightened to an appropriate torque. I hope this is helpful.