These are the small mods I've found have made a big difference
These two mods are ones that are talked about frequently (in the circles I hang around in at any rate!), and they may be of interest to you. Then again they may not be, but hey, what have you got to lose?
The 50's wiring mod. This mod is a simple change to the way the potentiometers and capacitors are wired which stops treble getting bled off as the volume of the pickup is reduced. Obviously, this will only be effective if you actually use your volume pots!
The Jim Dunlop Crybaby Wah true bypass mod. This mod is more complex than the 50's mod, so be sure you're comfortable with taking things apart before you attempt it. What this mod does is change a standard "off the shelf" Crybaby wah into one with true bypass, thereby stopping the guitar signal from being effected even when the pedal is off. This is useful in two ways - first, the signal is uneffected therefore giving a truer sound. Secondly, it means that if the battery fails whilst you're playing, the sound doesn't get cut off leaving you in an embarrasing situation!