If you really want to be serious about cleaning your amphetamine, then you'll need to do two washes/filtrations.
First, to get any nasty solvents out, you'll want to rinse the amphetamine with acetone or ether (acetone is probably easier to get, and if you use it, make sure it is cold to maximize yield -- ether is better though if you can get it). Since amphetamine sulfate is practically insoluble in acetone/ether, you will pour your solvent over the amphetamine (you could use a coffee filter and filter paper) and then just let air dry, then collect the crystals left in the filter paper. Make sure to rinse your solids another two times before leaving to dry.
Second, you are going to want to get out any acetone/ether-insoluble cuts that might well still remain in your solids. To do this, I would filter again with coffee filter/paper, this time pouring cold (but not icy) water over the crystals. Since amphetamine sulfate is soluble in water, pouring water over your solids will dissolve most of your crystals. Hopefully most of the cuts left in your speed after step one are insoluble in water and will thus NOT dissolve along with your amphetamine. Rinse twice with cold H2O. Now, you have fairly pure amphetamine sulfate dissolved in the water that trickles through the filter. Discard whatever solids may be in the filter/filter paper and take your speedy water and put it in a flat dish, setting it aside to the water evaporate. You could speed up the process in an oven on low heat (200 F max), although I recommend just leaving it to sit out over a couple of days.
You wouldn't need more than, say, 5-10 ml of solvent (whether acetone/ether or water) per gram of speed for each filtration (for purity you should filter 2-3 times in each step). Remember that it is better to filter multiple times with smaller amounts of liquid, than it is to filter just once with a large amount of liquid.
Once you have your cleaned amphetamine sulfate crystals, you can indeed smoke them, though it's not as good as smoking meth, and the sulfate salt can be a little yucky. Personally I would say you are better off snorting it...but give smoking a go and see how you like it.
[ 30 May 2002: Message edited by: fairnymph ]