It's a bit complicated because there are several different giveaways at various time to prevent overloading the server. What you do is make a list of several books you would like for a particular giveaway, in order of preference, and post them as quickly as possible when the thread goes up. If you don't win anything, you can enter another giveaway.
So you have to pick the time, make your list up in advance, and pounce. :-)