First figure out what kind of party it is that you're getting booked to spin and what is it that they want to hear. It can range new music, old music, everything in between. So you definitely have to understand what crowd you're going to face. If it's a top 40 commercial club, but the age range is like, you know, 21 to like 26 or 27, I'm not going to play anything that's too old that they might not remember or they might not know about. Even though I will try to incorporate some music that I feel that they should know about, but I try to usually keep it current because that's what they want to hear.
So whatever's on the radio, you can pretty much more or less try to track those songs down. You can also go online and check on various sites. I pretty much do all of the above. I'm like always searching, trying to stay on top of it all, and asking my fellow DJ friends, "Hey, what are you playing right now? What's popular in the clubs right now that I might not know about or I just haven't heard yet?"
You can also try to build relationships with people from various record labels. Nowadays it's a lot easier. You can pretty much just have them email you promo copies of songs that haven't really broke the charts yet, that are going to require some time to become really, really popular, but you know they're on their way.
No two parties are the same, so definitely keep that in mind. Whatever worked for your last party may not necessarily work for the next party, so you don't want to get into like a routine. You don't want people to predict what the next record is going to be.