i tend to pich my tarp with the back edge touching the ground, pegged in with some logs or soil etc to help stop the wind, if your in a wood the snow should not be too much anyway, depending how thick the wood is. keep the top edge high facing away from the wind, this will mean snow slides of, and you can also have a fire in front of the tarp if you want to, the army tarps are quite robust all round, so good choice....
if your looking at lots of snow then a ridge line will be required pegged down so its at a very steep angle, this will keep the snow of but will offer little room for moving around, but thats fine because you can open it out during the day if you like...
snow camping though, the best, my fave time of year is winter camping, nothing like having a brew in bed watching the world go by, the birds and animals twitter around with the gentle pitter patter of snow on the tarp.....
im out this year too, cant wait....