Most common mod is probably going from a 14/45 stock setup to a 15/45 setup. Lowers revs a smidge, making highway travel a bit less hectic. But people have gone all the way to a 16T up front, with a little grinding of the sprocket cover.
In the rear, there are a bunch of sizes that work fine, and some people do swear by a 15/42, 15/41, or even 16/41 setup, each of which will lower the revs for a given road speed just a little bit more. But - every little bit you go taller, the harder it is to get rolling from a stop in 1st, requiring more clutch slipping.
Good site to play with all of the combinations is:
www.gearingcommander.com
Some relevant threads with some reviews of different setups here on this site:
Sprocket Setup
DIY- Sprocket Swap
13t front sprocket
15/41 sprocket set up
16/41 Sprocket Review
/thread moved to new-gen tech area