Well, for me 20 gal
is only about 45% of my water, but as europas_ice says, I do stir up the gravel quite a bit and a lot of stuff does float around the tank, some of which gets sucked up in the filter as I clean and some in the gravel vac (I have a canister filter that I keep running while I'm doing it). Once I've taken out all the water I'm going to, then I just stir up the gravel, let the filter take the rest of it until it all settles . . . stir it up some more, let it settle, stir it up, let it settle . . . just repeat until I'm happier with it, then fill my tank back up.
When I'm all done, I give the filter a rinse out and I'm back in business.