You really must try to listen to what you're doing as you're doing it.  This is not very easy at first, as you'll be too busy trying to suss out why things aren't going as they should.  Also, if you leave the headphones on your ear when you're mixing the two tunes it can be hard to hear what it actually sounds like over the speakers.  Of course if you take the headphones off, it can be hard to hear if the tempos are drifting.  You've got to balance between these two needs; as you get better, you'll find you can do more and more without having to keep the headphones on, which means you'll be able to hear what your mixing really sounds like as you're doing it.

Being able to hear what you're doing is a great help, as as well as making sure the beats are in line, you can make sure that the aesthetics of the mix are OK.  Some tunes can sound awful together - specially if they're both melodic and they're not in key.  Also, if you're banging the cross-fader around to drop bits of one tune on top of another, you want to listen to make sure that it sounds good and that the levels are OK.  What sounds good and what doesn't is of course subjective and you should gradually get a feel for it the more you practice.