I have been doing this for a while now. Here are a few tips that I have found useful...if only I followed them more regularly....
1) Anticipate a harder, longer job than what you think. When giving estimates, aim high. I am not a pessimist by nature, but this is just the way it is... almost every project and repair takes longer than predicted.
2) When things seem too puzzling, when getting frustrated, walk away, at least for a while. Put it away if you can, let it sit for a while. Fresh eyes can make all the difference. So can a bit of research, and a schematic... and a good night's sleep.
3)Some things are not worth fixing. The tough part is to know when to walk away.
4) Test everything BEFORE you start taking it apart, and note all flaws, notify the customer if you find additional things wrong, BEFORE you take it apart. This keeps you from getting blamed for those additional flaws... it's a lot easier to blame the tech for an issue once he has applied a screwdriver and poked round inside your electronic instrument!
5) Only put the repaired item back together enough so that you can safely test it, then test it before you assemble it the rest of the way. This saves the time of complete disassembly when the unit fails. Not only will you be upset to find out you still have problems with your repair, but you will have to take it all apart a second time!
6) Shake it before you put it back together, rather than hearing that screw rattling around inside when you are lifting it off of your bench, all reassembled, thinking it's all done.
7) Mark every connector that you remove with a sharpie, so you are not puzzled about what goes where. Mark the sides of hardware with a sharpie too so you can easily remember where they go when you reassemble. Every time you take something apart, remember you are (most likely) the guy that's going to be putting it back together, maybe weeks from now. Be nice to yourself!
8) finish every job as much as you can before walking away from it. Why have a bunch of half finished things lying around?
9) Keep the screws and hardware in baggies, separated enough so that you don't spend a bunch of time figuring what goes where.
10) start with a clean bench and workspace for every new repair. It's worth it.
11) If you can't find something in the shop, clean up as you look for it. This way, even if you don't find it, at least you wound up with a clean shop.
12)Don't call the customer with good news until you have tested it 100%, and it is 100% together. Otherwise, the customer may be on their way to pick it up and you will be scrambling to remember how to put it back together, or worse yet you might be scrambling to fix an issue you didn't fix completely.
13) Never lose your cool, with the repair or in interpersonal situations either. If a customer is being unreasonable, it is best to be like a professional politician and stay "on message", never getting sidetracked or emotional in a negative way.