1. I read the synopsis:

Never would have guessed, have you? :D Of course I start with reading the synopsis and try to decide whether the story is interesting for me or not. It's an important step for me, but I never choose only based on this, especially that there are some very scant ones.
2. I read a positive and a negative review

It's very important not to read only one side. The best is to read as many review as possible and necessary to get the whole picture, but if I'm lack of time, I always at least read a longer positive and negative review, looking for aspects such as:
- What did they like/dislike in the book?
- How was the world building?
- What were the characters like?
- Was the author's style/writing good?
Every person is different and there are traits that don't bother some people whereas it do others so I also take thoughts on how much a trait would bother or not bother me.
3. I seek out an excerpt

Lately I've always read if I found an excerpt from the book and I did spare myself from a book I wouldn't have liked. Because although the first pages do not tell you how suspenseful the plot would be, yet you can conclude certain things, for instance the author's writing's style which is, at least for me, is a relevant aspect, and it definitely tells you about a bit how good the book can be expected to be. Moreover, you learn some useful information like; whose POV the book is in? All in all, this is something I would recommend you to do.
What about you? How do you pick your next reading?