* Listen to the customer, but don't ever trust them. They know things they will not tell you, and tell you things they don't know.
* Obtain as much vehicle history as you possibly can. Example: If the car was a bank repo, I know I may be looking for sabotage, as opposed to a problem/failure.
* Create realistic customer expectations. Get it out of the way right from the beginning that you are a technician, and not a magician, and the cost/time or repair will be dictated strictly according to the vehicle condition, and not by your knowledge of "tricks". There's no magic wand in the Snap-On catalogue.
* Never, ever, under any circumstances perform a bull**** repair, or service a part of any system in poor shape. No matter how much you save the customer, or how many ways you explain that they have opted to take an incorrect or inadvisable course to "fixing" their problem, when the **** hits the fan later down the road, you will not be thanked for giving their vehicle an extension on life that it didn't deserve. You will be blamed for all problems in their life, including their divorce, thinning hair line, and their cat having diarrhea. Give them back their keys and tell them they can do it right, or go take it somewhere else. The cheap customers always become the biggest problems.
* Always use service manuals and guides as just that; GUIDES. Like your girlfriend, they can omit very important details, and create some seriously high expectations as compared to reality.
* Keep a bottle of contact lens "multi-purpose solution" handy, whether you use contact or not. This is the perfect and doctor recommended solution for flushing out your eyes if you get anything in them. obviously, if you get a major blast of fluids in your eyes, yes, use the eye wash station, but for dust, particles, etc., the solution is much better for your eyes than water.