my rules:
ALWAYS have grass on you, just to mellow you out, shitty stuff works good too.
if someone/something upsets you, get the hell away.
don't ever trip on a whim; plan it out first.
don't trip alone unless you're positive you can handle it.
always remain calm. it will all be over soon. you're not going crazy.
don't try to fight the effects. drugs are stronger than you, and they will kick you right in the teeth if you upset them.
never, ever, ever eat spaghetti.

from the lycaeum's lsd archives:
Cars can hurt you.
You cannot fly.
It's never a good time to die.
taking your clothes off will draw attention.
Keep mouth shut at all times in public.
Although you may see things that are not there, you'll still be able to see real things. In other words, cars that are present in the street will not become invisible.
don't forget how to burp
only carry a house-key, some loose change, and your address in your shoe
nobody can tell you are tripping till you tell them "I'm tripping".
no matter how fucked you think you are, you'll eventually come down!!!

Another few points:
Sex with somebody you are not in a relationship with is a bad idea at this time.
Always make sure you have a 'safe' place to return to - somewhere you feel comfortable and private, and do not feel at all worried that somebody unpleasant or unknown will show up.
Have distractions available. Cartoons are good.
Have some way of getting food available - eating is pleasant.
Having downs to take the "edge" off the trip might be a good idea - alcohol or valium supposedly works well. (I have not tried this.)
Smoking a joint or drinking orange juice will give the trip a small boost - though smoking also cut a bit of the edge off