How to keep carpets clean?

I grew up in a house with tile and wood floors, and me and my husbands first apartment was all linoleum floors. Our second apartment is carpeted floors. We've lived here nearly 2 years, and it never crossed our minds to actually clean the carpets. The carpeting was always gross, so we threw tons of rugs down. Only thing we've ever done is vacuum, and we don't walk barefoot in the house or sit on the carpet (too gross). My coworker let us borrow his carpet cleaner this weekend, so we could clean the floor. It is the nastiest thing I have habpnds down ever seen come off a floor. I regularly clean the tiled kitchen and bathroom with a scrub brush and bucket of water (like I have always cleaned hard flooring with) so they are always nice, and not much dirt aculates since I clean them once a week). The carpet on the other hand, I'm just at a loss for words. My husband has gone over the same square of the living room 3 times (on his 4th time), and the water is still coming out black (it's getting a little clearer every time he empties it). Once we get it clean, what can we do so it doesn't get this nasty again? I am so disgusted. Obviously, no more shoes in the house, but what other measures can be taken? We don't have room for a carpet steamer, so we can't clean it once a month or anything. I do vacuum the living room a few times a week, since we have a parrot and he makes a mess with his seeds, and the bedroom once a week. Suggestions?