Why is my bike slipping gears? I have a 1997 Gary Fisher Tassajara that got a complete overhaul and tune-up at a bike shop a year ago.  (to the tune of $180, but some of that was new tires- I'm the original owner)  I ride between 4 and 8 miles on logging roads and easy dirt trails several times a week, and didn't have this issue until late Spring.  The slipping started infrequently, but has gotten much worse in the past two months.  It doesn't slip in the highest gear, and I can sometimes find a lower one that will work for me- but other times I end up having to put the chain back on mid-ride.  I'm a little worried about the chain or gears actually breaking while I'm out in the wilderness alone, so have limited my backwoods trail-taking until I get this issue taken care of.
When your bike was overhauled they likely replaced the shifter cables. The cables will stretch over time. The result is the derailleurs don't shift as accurately as they should. It is a fairly simple process to readjust them. There are many on line tutorials that will show you how to do this yourself. If you don't feel comfortable doing it yourself your local shop can help you.