Not every mechanic is out to cheat people out of their hard-earned money, but unfortunately a greedy few have given the industry a bad reputation in the eyes of some customers. You've probably heard stories from people who have experienced this. Or, even worse, maybe you've been the victim of one of these tricks yourself. Plenty of people have been taken advantage of, and it's still something everyone should be cautious about. I'm thankful to have never been scammed by a mechanic (although I do regret a visit to a particular dealership), but do have a few thoughts on how to be smart about getting car work done. Just to clarify, I won't be going into great depth about finding a good mechanic. There are lots of good resources and tips for filtering out the con artists, but sometimes you may not have the option (or the time) to find one you're 100% comfortable with.
Give a mechanic the impression you already know what you're talking about, even if you don't. You may need some help with this one. I'll explain. Let's say, for example, that sometimes your car won't start. If that's all you tell the mechanic, you have opened yourself up for a world of scams. Start by being as specific as you can. For this example, make a note of anything unusual you have noticed - do the lights still come on, do you hear any noises, does it happen when the car hasn't been used for a while? Here's where the help from a friend can come in. If you know someone who is more familiar with cars, tell that person your symptoms first. He or she may be able to suggest a couple of possible causes. Maybe he thinks it sounds like a bad starter relay, or just a battery problem. That way, you can let the mechanic know you've already done some research on the problem. Telling him, "I think my starter needs to be replaced, can you test that?" instead of "my car won't start" is a good way to begin.
Ask for quotes in writing before you allow them to do any work on your car. Make sure you know exactly what they are quoting too. They should know ahead of time approximately what parts, labor, and other materials (fluids, hardware, etc.) should cost, so make sure all of that (and an estimate of tax) are included in your quote. Don't be afraid to ask questions. Even if you think they are dumb questions, this will show the mechanic you're keeping track of all the details.
Once you have the quote, request a verbal explanation for each repair, and tell them you want to keep the old parts when they are done. Make sure you tell them not do to any unquoted work before asking for your consent. Shady mechanics have been known to do extra (sometimes unnecessary) work without the permission of the customer. It may be harder to get this commitment in writing, but it's not a bad idea to ask for it if you can. By now you should have communicated pretty clearly to your mechanic that you are not willing to be easily tricked, and will do what it takes to keep him honest. Hopefully, he realizes this before he's even done anything he can charge you for.
Once you're comfortable with the work that has been quoted, and have given the go-ahead, it's time to stick to your guns. If the mechanic calls you back to inform you that more work needs to be done, inform him that more questions need to be asked first. Get all the details you can about the additional repairs (why, how, how much). This may be a good point to get your friend involved again, just to confirm that the extra work makes sense. If you don't have a friend who knows cars, do a little bit of research on the internet if you've got time. It may raise a few red flags about the parts being replaced or the price being charged. There can be situations where a mechanic could legitimately find another problem while working on your car.