I have a few different (not necessarily distinct) practices, so I'll answer for each different style, keeping the same order for all the answers...
-What is the name of the type of meditation you do?
1. Shikantanza
2. Flow Arts (fire and skill toy manipulation. I'm personally quite fond of and relatively skilled with the diabolo)
3. Sleeping meditation (I'm told it's very similar to yoga-nidra, but I've come into the practice independently and I've never studied yoga-nidra, so I can't judge that)
4. Random-*** mindfulness/compassion/discipline/peace exercises
-What is the purpose, or the benefits of this meditation, both in theory and in your practice so far?
1. I think the theoretical purpose is to help come to profound realization, but what with the emptiness stuff in Zen as I've studied and practiced it, that means nothing. In my personal practice, I believe that this particular practice has helped me control my attachment and gives me a strong venue to practice right understanding.
2. The theoretical purpose of this is pretty academic: there's empirical research suggesting that Flow is all kinds of good for mental and physical health, but the practical purpose is that it's just fun. I consider Flow to be a capturing of the threshold that dances constantly between the ever-moving yin and yang, and Flow practice has given me time for sober examination of the esoterics that occur immediately approaching that threshold in a way that I cannot obtain using the directness of my normal prajnaparamita-driven practice.
3. I can't say anything about theoretical aims, because it's more of an accidentally-developed personal ritual than anything else. Practically, I can say that it's just a nice, relaxing way to wind down the day when I lay down to go to bed and it may contribute to how very well I sleep, since I deliberately calm my mind before even attempting the actual process of sleep.
4. Theoretical purpose is just to keep my guard up, keep the ol' brain-pan limber and warm, keep me constantly practicing, since I don't practice the other things too intensively. Plus I find it entertaining, like solving puzzles or something, just without the puzzle. Or the solving. So far as practical benefits are concerned, I can't honestly tell if there are any.
-Please describe what you do when you meditate (what you physically do with your body and what sort of mental techniques you utilize).
1. Cross my legs, usually in half-lots, fold my hands unto the Universal Mudra, spine straight, eyes open, focus on nothing, pay attention to everything and just.... post.
2. I mostly play with my diabolo or ride my motorcycle. Flow happens getting good enough at something that you can completely immerse yourself in doing it in a challenging fashion, that immersion is the flow state. So I practice old tricks, drill new tricks, jam the hell out with my diabolo, yo. Or I hit the Interstate with the bike. Both require all my focus to do correctly, and both are flow.
3. I do this basically only when I'm going to bed. I lay on my back, fold my hands over my chest, cross my legs at the ankle, and close my eyes. I usually take one or two big, deep calming breaths, and then I just relax. I sleep on my stomach usually, and it feels unnatural to do otherwise, so laying on my back allows me to relax deeply but still maintain alertness. I relax my body, my breath, and my mind. I let the thoughtstream flow until it settles into quietness, I immerse myself in the quiet of the direct experience of my quiet bedroom with the lights out, and then I roll over onto my stomach, stick my hands between my pillow and the mattress, and fall asleep.
4. Uh, I do whatever comes to mind. I hold mushin while working. I bring up an idea in my mind and try to see if I can hold onto it as clearly as possible while adding a second idea, continue to add ideas until it collapses. I try to see how long I can maintain maximum mindfulness to the outside world. I carefully and precisely experience love and compassion for every person I pass. I try not to lose The Game. Y'know, whatever.
-How often do you meditate, and for how long?
1. I fluctuate. Sometimes every day, sometimes once every couple of weeks. No more than five minutes or so at a time, though barring crisis, I think. I don't keep time. I sit down when it seems like sitting down is the thing to do, stand up when it seems like standing up is the thing to do.
2. Uh, when I play, I probably go for half an hour to an hour at a time, and on good days I'll do that two or three times in a day. But then I'll go three or four days without play at all.
3. Almost every day, about two minutes (I think--again, I don't take time. I take my breaths as I lay down, I turn back over when everything's still).
4. Every day, usually four or five times a day, I'd be surprised if the average exercise lasted more than sixty seconds.