Well, I don't think mixing drum'n'bass is that different from mixing house, trance or techno, theoretically. Every measure has 4 steps (although some kinds of jazzy drum'n'bass are known for having different structures), and you need to "fit" them in the pace of the previous tune.
The first snare is usually on the second step (although not always), and the first beat is almost always in the first step. After that, the second snare, as Lillith said, also tends to come in the fourth step, giving you another shot. So, really, the third step tends to be the only one "off", giving you plenty of opportunity to mix using the snares and the first step
“All I have learned, I learned from basslines.”