I've been experimenting with a little trick I read about recently. It seems to work reasonably well:
Just boil up your water in your billy can or whatever, add as much coffee as you like and let it steep for a bit, then flick a little cold water over the surface of the liquid (I just dip my fingers in some water and flick the drops); then tap the side of the billy fairly firmly a few times with your spoon or something.
The theory is that all the grounds should then sink to the bottom. Then you just carefully pour off the liquid into your cup.
As I say, it seems to work OK - you might get a very small number of grounds in your mouth, but so what?