I'll give you the biggest most important tip any dj has ever given me:
Get a stopwatch, and learn how to count beats per minute. It sounds boring and time consuming but eventually you'll be able to count beats in your head and get within about 4 beats to what you're actually hearing. I practiced with a metronome for a little while, i usually get around 2 beats or right on the money most of the time. Plus you can learn the BPM of all your favorite songs while you're practicing.
That makes all the difference in the world when you're right on. I can't stand dj's who try to speed up, or slow down a song just so the beats can match. That's horrible to me!
And when you mix live, always go from your slowest BPM song up. That way you get the slower songs out of the way at the beginning of the party.
I had a gig this past saturday and i tried something new. I zig-zaged my BPM by going up then down with different genres of music. It worked out pretty good.
I started with like 80 bpm with hip-hop, went up to like 87, then started a rock set, then when i got to like 118 or 120, i started an 80's set, then when my 80's went up to 138 i started a dance/rave set, then i came down the same way too. I wanna eventually get to the point where i can go up in one genre then come down in another then back up in a 3rd genre...make sense?
I hope you understand what i'm talking about. If not then let me know and i'll try and help out.
Good luck.