1.) Don't EVER start your KX500 without first putting on your riding boots. Otherwise you too may be the recipient of a gnarly gash down the front of your shin bone when your foot slips off the kickliver at the end of the kick swing and the end of the kick lever shanks the be-jesus out of your shin... allowing you to marvel in wonder at the sub-cutaneous fat deposits you happen to have just to the edge of that bone.
2.) Search the forums here for help on cutting-down vibration in the handlebars. Your fore-arm pump will thank you.
3.) "With great power comes great responsibility." Be gentle to your riding buddies behind you on the trail. Shower them with rocks no more than twice per outing. Well... ok, maybe three times is excusable. (insert fiendish grin here)
4.) Back tire needs to be 120 width or GREATER. Preferrably GREATER. Search forums for other riders suggested models.
5.) Familiarize yourself with the exhaust mounting point at the top-side of the expansion chamber where it mounts under the gas-tank. This will loosen, or the mounting plate may crack a bit if you have a fall and the pipe gets pushed upon.
6.) Re-pack the silencer. d**n these bikes are loud when the packing is slim and none.
7.) Get used to periodically stopping on the trail and waiting for your friends to catch-up. It's a good habit to get into. Especially if they are lesser-skilled than you, while at the same time riding something else besides a KX500 trying to keep up with you. If they are smart, they will figure out they need to slow down, if not... you may be called upon to ride back to camp and fetch a truck.
8.) Get used to slipping the clutch a lil bit when climbing-up rock-infested technical uphill sections. The clutch slipping will increase your controllability by light years.
9.) Personally, I highly recommend replacing the stock seat-cover with a full gripper seat cover. As a cheap alternative, I also had good luck spraying a thin "dusting" of spray adhesive over my stock seat. You need all the help you can get keeping her a$$ on the seat of the beast.
10.) When the suspension is setup properly. You can blast across vast stretches of age-old massively-formed-over-time-by-a-million-other-riders California/Baja style Desert whoops faster than you could have ever believed possible.
11.) Stop frequently and take brakes while on the trail. You're now riding a seriously powerful machine. Do not make the mistake of allowing your fatigue levels to get the better of you. The results are expensive hospital/surgery bills and additional metallic parts being strategically grafted to your body.
12.) Welcome to bragging rights.