Well, mixing technique : pretty obvious you must have solid beatmixing skills. So try to make as few errors possible (in fact they should hear the transition between songs the least possible). It also helps if you throw some tricks in your set (switching beats, using acapella's, scratching,...), BUT : only if you're quite sure you will succeed to perform the trick flawlessly. It's better to concentrate to a smooth mix, rather than a mix full of tricks but that sounds like crap. They'll also judge according the difficulty level you chose. If you succeed to make a good harmonic mix of two very cheesy trance tunes instead of just mixing the beginning beats of the new track with the ending beats of the old one (so just plain beatmixing in fact), you'll get a better score (obviously!).
About stage appearance and communication with the public, they go together... You'll understand that if you make a perfect mix technically spoken, but you scare the crowd away, the judges will penalize you for that. Make sure the crowd has some good time too, cuz the judges will look at the response and give a grade accordingly. Be sure you have a confirmed look (nothing worse than a dj that looks and acts like he never touched a mixer in his life before!!!), be cool and the least nervous possible (Relax, Max...).
Be mister nice guy, make sure everyone remembers you for a long time (no not by mixing like crap or by acting like a nerd).
If you have a mic at disposition, I wouldn't use it too much. It's allright to shout some things like "put your hands up in the air", "are you ready (to rumble)" ("mother****eeeeeers.. goooooooooooo"
, if you know the tune by Avancada, you'll unbderstand what I mean), but this should be very sporadically. The music primes, don't forget that, if they wanted a talk show they'd watch Jay Leno on TV
.
Now about the mix : many people prepare a set for competitions. That's better for the technique side of it, but by using a set you won't be able to play FOR the crowd (if they don't like your set, you're out). Always have a backdoor, mixing on the fly, so you can always see the crowd likes it.
