Well, I was going to chime in on the rather brutal take-down of poor ol' gooldteefs, but it seems others have already done so... so I'll let that part go.
I have yet to run a DMT experiment, but I have experimented with other hallucinogens (including extreme doses of LSD). I seldom experienced any negative effects on acid. In fact, people wanted me around during their trips because I helped them feel safe. However, 20X Salvia was a total nightmare! I have trouble even speaking its name!
There is no fool-proof preparation method that insures you will have an enjoyable experience when experimenting with any psychoactive compound. This is all "chemistry." The fact remains that you are taking a powerful drug that alters your perception to the point that it is tantamount to psychosis. If some of the things you see are not what you want to see, well, you're pretty much screwed. You're screwed for the halflife of the compound and that's just the way it goes.
I will agree that having a good attitude, no apprehension and a safe, comfortable environment are all important, but these are all just common sense things YOU can do to make sure you aren't doing anything that may contribute to a bad experience. If you are going through a divorce, filing bankruptcy or just lost your job... that's probably going to be made manifest in some horrible way after taking your drug. On the flip side, you could have just won the lottery and scored with Adriana Lima and STILL have a horrible experience on ANY psychoactive drug.
There are soooooo many variables that come into play that nobody can accurately claim they have a fool proof method for having an enjoyable experience (or can condemn someone who has just experienced one)! Safe use of Hallucinogens is a complicated balancing act because, once again... it's all chemistry both outside and inward.
-Birdman
P.S. Personal Tip: Have a pre-determined goal in mind before you launch into space. During the days before you experiment with any psychoactive compound, do a lot of thinking about a complex questions you may have regarding life (like, "Why do I exists?" "Is there a God?" "Why do we suffer in life?" ). I have found that if you genuinely reflect on these issues and condition your mind a few days beforehand, your mind will reciprocate in a more friendly and fulfilling manner. ...It's like practicing your music for a few days before you go on stage.
Your brain is a highly-refined tool used for "discovery." It wasn't stuck inside your head just to entertain you.