There's a few ways.
Some DJs basically pick a bpm and stick to it for the whole set, for example most drum n bass tunes are written around 160 bpm, so DJs basically sit on this for all of the set. You can drop in half-time tracks (ie. 80 bpm) to break it up a bit if it's getting monotonous, or you want to have a bit of a lull in your set before cranking it up again.
With regular pitch/tempo shifting, you should be able to change tracks from at least +/- 80 bpm (I think it's probably more, but it's been a long time since I've done any of this stuff), so you should be able to get most tunes into a similar bpm, either at the same tempo, or half-time.
If you want to play tunes with radically different bpms, this is where knowing your tunes comes in. Basically you want to mix in in the bits of songs where there is no beat. Or gradually slide the pitch shifter down to approach the tempo of the slower track, or up for a faster track (while you have the track you're mixing into going in your headphones, of course).
This might seem obvious, but generally if you're playing a whole bunch of songs with different tempos you want to go from slow to fast over the course of your set. People can get quite put off if they've been rocking out at 180 and suddenly you play a bunch of things at 100 or something.
