1st off the main thing you need to learn is how the beat patterns are, and when, and how to mix them. beat matching is pretty difficult at 1st but the main thing is DO NOT RELY ON BEAT COUNTERS !!!, mainly because your ear is far more accurate then a beatcounter. Beat counters are nice for helping you find out roughly where you are BPM wise, but not for beatmixing. My biggest tip on beatmixing, is 1st start the 1st song. listen to how its beats go. Listen to the songs a few times, and get used to the structure of how the song begins, and breaks. Then get out a second song with a simular beat pattern preferably. And cue to the 1st bass beat (ussually a strong thump or boom followed by a snare or SFX). now rock the 1st bass beat (done simply by scratching the 1st bass beat or bar with the other songs beat). Doing this will momentarily match the two beats but eventually the beats will become offset. Now is the time when you start messing with the pitch fader. Repeating the steps above (rocking the beat, etc). adjust the pitch until the song stays synced nicly. *note that the beats will always have a bit of an offset, so you will need to learn how to "bend the pitch". Doing this has various methods, but me being a digital dj ill just have to explain it the best i can. On Vinyl the best way to bend pitch is to lightly touch the edge of the record. Doing this will momentarly decrease the pitch to get the beats back on track. But if the song is to slow, and the beat is draging. You will have to bend the pitch up. the main method i have seen with this is slightly nudging the label on the record forward. Doing this will increase the speed momentarily to get the mix back on track.
Well this is pretty much the only way i can describe beatmixing, for more help feel free to contact me at: firstname.lastname@example.org
and maybe i can give you a live webcam tutorial.