Any advice on leg numbing during sitting?

It happens at about 20 minutes every time lately... either in half lotus (a traditional position that I would like to honor), or with one leg laying in front of the other, or Indian style... doesn't matter, my left leg goes numb. Sitting up in a chair doesn't seem conducive to a meditative state, and laying down I just fall asleep. I'd really like to do some intensive and very long blocks of meditation (advice on this would be appreciated also) but I'm not sure how to approach this severe distraction and get past it.