Dont accidently stick your fingers in disc brake rotors.
Dont accidently cut your fingertip off cleaining a fixed gear chain.
When doing a repair job layout all the tools you expect to use beforehand, so you dont have to waste time looking and you wont loose things in your own mess.
Grease in a tub gets contaminated, grease from tube not so much.
Depending on job, its not always lefty loosey righty tighty
Keep a long steel "fixin pipe" around, you can put it on the ends of wrenches and get tons more leverage
Channel Lock Pliers is an awesome BB tool for one piece cranks or anything that requires a really big crescent wrench. (at the shop we call it the Park CL-FFET, fix f***in everything tool)
Lube everything that turns.
When tuneing up a bike, start from the front and work your way back so you dont miss anything.
Turn off cell phone when building wheels.
If it dosent work after trying to fix it for several hours, cuss at it.
If your tube goes flat, rub it more. (just kidding)
When adjusting cable tension on a rear derailer, shift to the hardest gear and run the barrel adjuster all the way in. Then shift one click, and turn the barrel adjuster until the chain jumps up a cog, then give it half a turn for good measure.
If indexing is off, your derailer hanger is probbly bent.
you can get a wheel decently true on the bike by tightening the brake calipers down as a guide.
Finally, try to refrain from beating on repair bikes with a hammer in front of customers. GOD knows that eventually you will need to beat something with a hammer or cut something off with a hacksaw, but the customers dont wanna see that kind of thing.