05/21/2009: "Suspension Setup by Evolution Suspension"
This really isn't a mod, per-se, but I think this is some of the best money you can spend (on the motorcycle itself) to help make your riding better. Get your suspension properly setup for YOU. You can't really do this by yourself, you need at least a couple of friends and someone that's got some experience or knowledge of how to set the suspension.
I have both, but didn't really have anyone around to hold the bike for me, so I took it over to Rob at Evolution Suspension.
For a mere $40, he was able to determine I didn't need new springs (luckily) and setup my suspension for me. Turns out, Duchess didn't have enough preload in the front or the rear, but it was really bad in the rear with an extra 10mm of static sag. Apparently, the previous owner never set it.
Big thanks to Rob for getting my suspension dialed in for me.