It's not that hard to find a trustworthy mechanic--until you're desperate. It's worth taking the time to look for one before you're in dire straits. Have a good mechanic regularly maintain your car to help keep surprise breakdowns to a minimum.
Ask friends who have your make or model car if they would recommend their mechanic.
Get a recommendation of certified mechanics from an auto club like AAA, if you're a member. You can also find mechanics with valuable certification from the National Institute for Auto Service Excellence (ASEcert.org).
If you have a specialty vehicle or sports car, try to get a recommendation from a local auto club--the Miata Club, for example.
Find a dealer that has the most service awards from the manufacturer. Those mechanics have been manufacturer certified.
Check with the Better Business Bureau (bbb.org) to learn whether a garage has any strikes against it. Ask how the shop charges before you have any work done.