And I usually watch one trailer, because I like to keep spoilers to a minimum. If it is a series I already know, I don't need previews, but if I am going into something, I read IGN's review.
If it's FIFA then I usually buy it... I do watch a few trailers first though.
I have bought a game without watching any trailers or reading any reviews... Didn't turn out so well... I had no idea what I was doing and the game was awful. Luckily it was only 98p or something and I traded it back in within 10 days for a full refund.
Usually I'll see the trailer, or have played it before. That's usually at a friends house, not really anything demo-like.
I try not to make bad purchases or impulse buys.
Pretty much all I do before I get a game is read the plot. If it sounds like something I'd like is basically my deciding factor. I'll occasionally read a review or two about one, but not as much as I probably should. If it's one of those pricey games though, then I do a little more homework on it. Just wish they'd bring down the prices of the new releases at least. Which is probably why I wait a while for the price to drop too. ^^
I very rarely buy games. The only games I buy are those that I am likely to play online more often. If I just want to play the game campaign then i'll rent it from LoveFilm. So for me it's based on multiplayer and my interest in it, so I don't really watch trailers to decide.