i've never had a "bad" trip and done plenty of lucy in my day..
saying that, two of the most important things are set and setting. only be around people whom you trust with your life, and in a happy place, w/e that means for you..
don't be around any negativity, and if your mind starts to go to a dark place, try changing either the set or setting..
if things start to get pretty heavy, i always remind myself that i'm on a heavy drug, and it will wear off soon, and it usually does the trick, at least enough to get me back in a happy place, not that i've ever really had a "bad" trip like i said earlier..
oh yeah, drugs like lsd intensify your mood, so if you're in a bad mood, or a sad mood, or a dark mood, it's not a very good time to take lsd, or pretty much any drug ime, but hallucinatory drugs are even worse to do if you're not in the right frame of mind before taking them..