What kind of shifters are they? What kind of bike is it? What kind of shifters and derailleurs does it have?
It's possible that your problem is not friction shifters in general, but those particular shifters. Maybe you need a nicer set of friction shifters (I prefer ratchet shifters). Nicer friction shifters means not having to change anything else (assuming everything else is in nice working order).
I guess we could give better answers with more info about the bike, though.