I for one am not exactly a "car guy" and every time I need a water pump replaced or timing belt repaired I head right on over to the nearest garage and turn over the keys to my beat up,old Ford Escort. Their have been many times I have just simply payed for services having no idea if the price was fair or if I was getting screwed over by some jerk who figures I would never know they difference, and of course there have been times when I just though to myself, How could this cost so much. Here's a few tips to make sure your not getting ripped of by your mechanic.
1) Ask ALOT of questions, make sure you know exactly what parts and type of labor your car will need.
2) If possible, wait there while the repairs are being made, this makes it harder for them to charge you for unnecessary repairs.
3) Request to see the part that is causing the problem and have them tell you why it is causing a problem to insure that they can't just clean up the old part and pass it off as new or install a new part that didn't even have a problem to begin with.
4) Do your research ,If you know whats wrong with your car, you should know how much you should expect to pay for parts and labor before you walk in the door, sites like repairpal.com make this very easy.
5) Shop around, I find that estimates can vary by hundreds of dollars depending on where I am, but also remember you get what you pay for so don't always jump at the cheapest price, alternatively don't be stupid enough to go with the highest estimate either, shoot for the middle.