a demo tape is exactly that: A demonstration of your DJing ability.
So whatever tracks you mix, aim to do a SEAMLESS (i.e. no crappy beat(mis)matching or skipping tracks) mix of about 30mins. I’ve found that 30mins is the preferred length WITH A TRACKLIST. It’s likely folks will listen to the first few tracks to hear what your mixing style is like and if they’re familiar with the tracks just look at your listing to make heads or tails of you.
Also if you’re sending the file over via e-mail (so either a link to a site like soundcloud or send it through a service like yousendit) don’t have the file be HUGE, it’s kinda annoying. So recording a set then exporting it as mp3 at 192kbps is usually good enough to get the message across (although sometimes people do it at 160VBR)
Good luck. 
