I choose by what genre/mood I feel like reading at the time. I tend to switch around a lot, especially if I just finished reading something kinda depressing-- like
Empress (or the third Temeraire book). Then I make sure to read a humor/light-hearted book next so I don't get too depressed.
I generally don't read a series all at once, just because I tend to get burnt out really quickly. But sometimes if the series is really exciting, like the Temeraire series (before I got to the end of the third book, that is), I'll push through and read at least a few of the books in a row. Just depends on how I feel.
(I hope that wasn't too long-winded!)