use a paint brush for cleaning off dirt. get the chunks off and then put some silicone shock oil on the brush. it softens the rest of the dirt and makes the plastic parts and tires shiney, lightly lubes the cvd's, hinge pins, ball cups... I wipe off the brushed on oil with a towel. it works great and it doesn't rust your hardware. Every once in a while I get shock oil in a kit that isn't labeled andd this is what I do with it.
make sure the holes in your rims are working and large enough. most rims aren't bad, but sometimes you get a plugged hole and it will mess up the handling.
pick upgrades that are actually upgrades. aluminum is heavy, graphite/composite parts are often more brittle. stock plastic in most cases (off-road)is your best friend.
when using brushed motors, give the brush leads a little tug before each race. it helps to keep them moving freely and greatly reduces the dreaded hung brush.
glue flat rubber washers(hardware store) on the tops of your body posts, it will prevent the posts from scratching threw your paint.
Get steel toed boots and a green mohawk. It won't make you faster, but fewer people will yell at you when your marshalling.
Current active RC's; Brushless Xray XB8, CRC Pro10, Tamiya M03, Xray M18, Losi Micro-T. Inactive list; Losi XXX-S, Duratrax Evader st, Traxxas TRX-1, Traxxas E-maxx, Brushless Ofna 9.5, Corally sp12m