Any substance that'll change an 800-pound bear's mind will sure as heck do some harm to a comparatively puny human. If you accidentally spray yourself, here's what to do:
Manufacturers and chemists recommend lots of soap and water, which usually isn't available in the backcountry. Plus we've found that splashing on water spreads the capsaicin and burns more skin. If you're near a creek, submerge whatever part is affected, and try to do so for 10 to 15 minutes.
If it's in your eyes, flush with lots and lots of water for 10 to 15 minutes. Remove and discard contact lenses.
If you have vegetable or olive oil for cooking, rub some on the affected area to dissolve the capsaicin, then flush with water.
Don't apply lotion or cream of any kind-even suntan lotion-to skin that's been sprayed. You'll only reactivate the capsaicin and intensify the burn.
Try to relax and wait it out. Hard to do, granted, but the burning will subside after an hour or so, and should vanish in several hours.