How do I stop my brake from sticking?

A friend brought me a bike with this problem, and after halfway disassembling the brake, I realized that the problem was that there's a little plastic sleeve that goes over the end of the spring, so that it can slide easily when the brake is applied. You can see where it is circled in red in the picture, after being put back to the right place. That sleeve had come out of position, so that spring was dragging directly on the brake body - much friction, not so good spring action. Once I put it back in the position, everything worked more easily. Just posting here since this is the top hit for "caliper brake sticking bicycle", to maybe save somebody else 20 minutes.

Lubricate everything, all moving parts of the calliper, inside the housing etc. Wipe excess off. Use a thin penetrating lubricant (tri-flo or similar, not WD40) Be very careful not to contaminate pads, rims or disc rotors though.

I once had a sticky cable disc brake calliper (BB7) which I thought had a weak spring. I was putting of the maintenance because I didn't want to get into the spring issue. Anyway I rode with a dragging brake for a long time. Then one day doing other maintenance I quickly oiled the calliper and it completely fixed the problem. An easy solution that I had put off for too long!

A few things off the top of my head:

1. Not enough brake cable tension. This is very likely if it's fairly new and hasn't been tuned up -- brake cables stretch over time, especially when new. The amount of stretch can be more than the barrel adjuster can compensate for, so you would need to actually pull more cable through the cable clamp on the caliper.

2. Broken (or worn out) caliper spring. Less likely, but you should be able to see if it's broken, depending on the model of brake. Your LBS might also be able to replace the spring rather than buying a whole new caliper.

3. Frayed cable or housing causes cable to "catch" and not spring back. Might be harder to find, you'll need to inspect the cables and housing for damage (may need to pull the cable out completely). The good news is that these are relatively cheap & easy to replace.