Since this came up I thought I'd share what my crackhead friends here do:

They save the stems in a glass jar until its full. Then then put acetone in the jar, close it shake it up for a while, and stick it in the freezer. It stays there a while, the longer the better they say. Then it gets strained and poured out onto some dish. They wait for the acetone to evaporate, and usually take tobacco and scrape the oil off the plate, then right into the bong.

Now, tobacco is definitely not healthy, but what do you think about the acetone? I think it leaves some residue, right?