Know your tunes. Inside and out. This will allow you to know what the perfect place to mix is. Obsess over finding the smoothest fade in and fade out. Nothing sweeter to a trained edm ear than a perfect smooth transition.

+1

good thread.
is their a rule on build ups?
i have herd many dj's mix edm and it seems like their whole mix is a build up
it really gets annoying

You can't always build up, you should tear it down as well once in a while, to give your audience some time to breath :o). But keep it interesting and make your buildups/teardowns smooth as well. You can hit the audience in the face with a high-energy track to hype them, but you can't keep slamming them from chill to hyped and back to chill constantly :o).

Balance and smoothness

how long should a edm track be played?
:) edm no0bie

For as long as it's interesting and you or other people want to hear it :o).
There's no rules, except maybe 30 second slam mixing... that might not get you far (not impossible though, Greg J pulls it off :o).

There's electro-house tracks that sound great for 1 minute and get boring after, while there's 12 minute techno-tracks that vary very little and you still want to hear every second of it. Ever heard Underworld live while they still had Darren Emmerson? They could tweak a track for 10 minutes and you would still not be bored, and they blended one track into the other without you even noticing so after the concert you'd feel like you heard only 1 really long but great song.

Anything goes, as long as you can pull it off and keep it interesting.
That's the nice thing about music, there are no rules (except for copyright of course *barf*) and that's what keeps music interesting and keeps it evolving. As soon as somebody sets rules or finds 'the' formula, it gets generic. Perhaps not bad per se, but not very exciting either.

Find some good transitions, find a good blend, work on near-perfect beatmatching and EQ-skills and just experiment, see what the crowd likes, how far they let you go. You will probably have to drop ClubBangers, but you can switch it up with some unknown stuff to, and if you do it right, you can get away with it and people will appreciate you for the variation.