I'm a little late to this thread, but I'll wade in... It depends on the event. If you do fund-raiser events (like I do a lot of) with cocktails, dinner, speeches, and social time, I can almost script the entire night. If it's a corporate event (like an office party), I have a bunch of playlists (crates) of specific types of music that I might play (i.e.; Country Slow Dance, 60's R&B, Jazz Vocal, 80's Pop Fast, Etc., Etc.). The lists are laid out in such a way that the tracks on it flow well together, but I might play 3 or 4 from one list, then move to another list. It kinda depends on what the crowd is diggin'. If I get a request, I slide it in with complementary tracks from one of the lists. Every once in a blue moon, I'll think of something that isn't on any of my lists, and I'll drag it in, but mostly I stick to what's on my playlists. I have too many tracks in my library to just wing it without narrowing down the choices, 'cause I'm old and my memory isn't as good as it used to be, so the playlists make it appear as if I'm not as feeble as I actually am.
I generally create a few new playlists specifically for each event, but some lists are constant (like "Line Dances" or "50's Rockabilly" for instance). This MO works for me.