I believe its good to be in a good state of mental well being before hand (I.e, no depression/schizophrenia).
As well as knowing that you WILL come back, no matter how far you blasted yourself into the cosmos of substances, and no matter how bad the trip it will end, eventually.
Also, YOU control your trip more than anything. You can be in a totally alien/frightening environment and have a great time, it all depends on how you angle your thoughts, so to say, at the time.
Those are my general 'rules of the thumb' with any psychedelic/substance.