One possibility that hasn't been mentioned so far (probably because it's a given for the experienced riders who have responded to your thread) is that of switching the current freewheel or cassette, as the case may be, to one with a more closely or more widely spaced range of sprockets.
If you've every ridden hard and wished that you had another gear just a little higher or a little lower, you're a candidate for a sprocket swap.
Note, though, that while switching to a smaller sprocket range should require nothing more than a bit of gear adjustment, if that, switching to a wider sprocket range sometimes necessitates installing a longer chain, a wider-range derailleur, etc.