A tune-up is the first step. And, if this car is has fuel injection, clean the injectors. On a car this old, there are lots of things that can be worn down, out, or through. Don't replace anything that doesn't absolutely need it, because replacing all the worn parts on an 85 Buick will cost more than the car is worth.

Also, for the same reason ( unless you have a relative with a garage) you should do most of the labor yourself.