Familiar with author
For me this is quite oiften the deciding factor. For example I recently read a Robin Hobb book and have since gone out of my way to get as much as her stuff as possible. Although the reason I picked it up in the first place was due to wanting something different to read and I saw a copy on the bookshop shelf...
Specific genre or sub-genre (F, SF, military SF, sword & sorcery, etc.)
Although it should not be, this is often a deciding factor. As I get older I find that there is less time for me to read what I really want to, let alone material that i should read but does not appeal so much, so I often end up sticking to what I know I like. Very naughty I know, should be trying to expand my horizons and all, but genrally I stick with the Fantasy and Science Fiction, of all types.
Cover and Presentation
There is of course the old adage Never Judge a book by the cover, but I sometimes do. if I'm looking for something new I'll often go into a store and its the covers that draw me to them first.
cover blurb, preface, quotes from other authors
Following on from the previous - if the cover takes my fancy the next thing I look at is the blurb, and if there is a comment by a writer I happen to like then all the better. But never judge on cover alone.
Content (flipping through book)
I never, ever do this
Recommendations from others
Most of the recommendations I get come from after I happen to say I've just started reading soemthing, and only then do my friends tell me I should have read sit sooner because it's great, or that i should not read that, it's better off being used as toilet paper.
Since joining Chronicles though, I have seen all the recommendations flying around and that has influenced a few choices!
(As if I did not have enought to read!)
Reviews (on Amazon, here, etc.)
I never really look at reviews, as taste is subjective. I've seen books I love get savaged and ones I thought terrible highly applauded, so i do my best not to look at the reviews
Other?
Quite often friends give me books for birthday and Christmas. One friend in particular is very good at giving me things that he feels are a little different, or that I need to read. once given, no matter how strange they me I am duty bound to read them. Some day.
And another reason is research. If there is something I am writing or working on, or just take an interest in I will pick up a few books on the subject.