I've used got one of these bags. In the UK they are known as 'millbank bags' (as used by the army). They are basically a thick canvas sock that you fill & hang up over your billy or water bottle. They can filter about 2L (2 quarts) at a time.

It works really well: even the muddiest water comes out clear, and you can add charcoal inside the bag to remove any unpleasant odours or tastes before sterilising it. Not the fastest way of filtering (takes about 10/20mins each time), but very light, efficient & will last forever.