Two short bits of information. First dont screw around wit this. Set and setting. Understand what will happen and better get an experienced trip sitter you trust. Give at least The Psychedelic Explorer's Guide a read.
And while my urge to drink is still completely gone you have to do some work when it comes to depression. From personal experience i can say it works on multiple layers. There is
* Giving you a short term point of reference you can later aim for (experiencing how not being depressed looks like)
* Making changes to your entrenched thought patterns more easy in the short term
* Allowing for more self reflection (i later learned the cognitive behavioral therapy aims at something similar)
First point also got me to look into meditation. Eckhard Tolles Power of Now describes the same experience through meditation. In the end its no surprise that Timothy Leary found the Tibetan Book of the Dead a great trip sitting manual. See Sam Harris Waking up for a rational explanation.
edit: After talking with a few people who had bad trips my personal pet theory is that your likelihood drastically increases if you are overly attached to your own perspective. To the degree that you cant really imagine that your perspective on reality could be wrong. Not that this is safe otherwise, but be very very careful if that describes you. You are the kind of personal for whom anything on the spectrum to ego death might be very unpleasant.