How you mix your tracks is as much what makes you an individual as the tracks themselves. Technically you have a few choices in the methods you use though as with any genre. Wherever possible the tempo should be matched during the mix though. The key of the tracks being mixed is just as important as the tempo, unintential key clashes can spoil the flidity of a mix. I have a freind who can't beatmatch very well but he has done some awesome mixes simply blending the ambient/breakdown sections in tracks with complimentary chord structures.

For Drum and Bass, Personally I like "throwing faders" (great phrase by the way Reborn, I might have to get that on a T-Shirt) wherever I can't beatmix intros. "throwing faders" also sounds great pre mix to intoduce or even tease the crowd with a track before bringing it in. I used to drop in a bar or two of a big crowd pleaser a few times many mixes prior to playing it in full much later on in the set.