it varies wildly depending on the gig. some performances are really low pressure, sometimes i'm playing a genre i'm really familiar/confident with, and sometimes i get booked for a short set, and in those cases i might not prep much if at all. my camp also used to run a weekly internet radio show (hopefully returning this spring) and i would hardly ever prep for those. usually i would just play all of the new tracks i had purchased that week, and those sets would kind of function as the prep or review for gigs outside of the radio show.
if i'm playing a high profile gig, a genre i haven't played out in a while, in front of a big crowd that's never seen me before, or something along those lines i'll often prep quite a bit. this could include identifying the key of all new tracks, experimenting with which ones sound good together, actually practicing instead of just winging it, setting loops, adding useful comments so i know if a track does something weird in the middle or has an intro i'd rather skip, etc.
it's funny how prep can sometimes work for you and sometimes work against you. i had two gigs in hawaii about a month ago, and i prepped heavily for the friday night club gig and not at all for the saturday night beach party. i wasn't even sure what genre i was going to play on the beach until about 10 minutes before i went on. the set on the beach was by far my superior performance of the weekend. i think sometimes prepping too much can stress you out and make things feel overly rigid. the show starts to become more about impressing people and less about having a good time. it's definitely a fine line.
i also have to agree with mdmahead, as i'm totally a cross-genre dj and this makes me more inclined to prep since i rarely play any style consistently enough to make it feel like second nature. i have a friend who uses a controller and auto sync in traktor and plays almost exclusively minimal techno. it's kind of hard to fuck that up - the software and the predictable structure of the tunes almost do the work for you. most prepping beyond the initial assessment and purchase of the tracks would be a waste of time.