Have with you at least as many cigarettes as you generally smoke in a day; however, if you are inside, be extra careful about what you do with your cigarette butts.
Have water readily available.
Remember: you are on drugs; everything is supposed to be weird (this can be a lot harder than you might think).
With acid in particular, smoke some pot if you're getting too edgy (I usually avoid very high quality "kind bud", since I don't actually get high while tripping, and the low cost stuff works just as well to mellow me out).
I sometimes like to keep a little piece of paper with "Don't Panic" written on it. This is some good advice (whether tripping or not) I picked up from the Hitchhiker's Guide to the Galaxy.

Something fun to try next time you trip (that my friend wixard told me about): get a glowstick, poke a hole in one end, and tie a string to the other. Go somewhere dark, and swing it around your head. You will see a glowing stream coming out of the glowstick. When all the liquid is drained, look around: you will be in the middle of what looks like a glowing starfield (This works even better in a small, enclosed area, so the "stars" get on the walls as well as the floor).
More rules:
Rule of thumb: If you're not sure whether you want to trip or not, don't.