Although it differs from the music genre you play.. I find personally using the low and mid faders can do the trick almost every time. With of course the right timing.
Matching the BPM's and switching over with the other songs beat blended in to the current track playing
Many different ways can be effective, the most important thing is to make sure beats are in sync and you find a good transition point. The key is to make the music flow seamlessly throughout your set. Try to avoid train wrecks. Practice, practice and more practice.
