If you take your car to the shop for regular maintenance or even a small repair, odds are the mechanic will come back to you with a laundry list of things they want to convince you to do before you leave. It's a difficult situation, since your car is already in their hands, but there's no reason you have to feel like you're being held hostage. Get written estimates before authorizing any work, then call around to other garages and find out how much they'll charge for the same thing.
Ideally, the best way to save a bundle on car repairs is to do them yourself (you'd be surprised how easy simple maintenance tasks really are, and even some not-so-simple tasks are easy once you've done them once or twice), but if you can't or the issue is over your head, don't feel like you're trapped just because your car is already on the lift. Many states have laws stating that you have the right to a written quote or estimate before they begin any work (although some garages ask you sign that right away as soon as you drop off the keys, so be careful) and even if there's no such law in your area, let them know you want written estimates for any work they want to do before they do it.
Once you have the estimates in hand, call around to other mechanics or dealerships in the area and let them know the situation. Tell them you have a quote for X-amount of dollars for Y-work to be performed, and ask if that's how much they'd charge for the job, or if they can do better if you brought your car to them. They may have a special, or be willing to undercut. In either case, now you're the empowered party, and you can negotiate with the original mechanic, or take your keys and your car across town to the cheaper one.
If you're not sure whether the repair you're being quoted should cost as much as a mechanic says, this is a good way to research labor and part costs while searching for a good repair shop at the same time. Worst case, you can visit previously mentioned Auto MD to research repair costs before you set foot into a shop.
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Photo by Robert Couse-Baker.