First, be smart from the very beginning. Pulverize all teeth, burn off fingerprints, and disfigure the face. Forcing a DNA test to establish identity (if it ever comes to that) might introduce the legal/forensic hurdle that saves your ass down the line. An unidentifiable body can, in a pinch, be dressed in thrift store clothes and dropped in a bad part of town where the police are less likely to question it. I don't reommend that disposal method, I'm just saying an easily identifiable body is an even bigger threat than the opposite.Suppose you killed somebody... (Thanks, Salim!)
Assuming you have it inside a house where you can work on it a bit, the first thing you want to do is drain it of fluids. This will make it easier to cut up, and slow decomposition a little bit. The best way to do this quick and dirty is to perforate the body with a pointed knife, and then perform CPR on it. Cut the fronts of the thighs deep, diagonally, to slit the femoral arteries. Then pump the chest. The valves in the heart will still work when dead, and the springback of the ribcage can put apply a fair amount of suction to the artria. Do this in a tub. Plug the drain, and mingle lots of bleach with the bodily fluids before unplugging the drain to empty the tub. This should help control the stench of death, which would otherwise reek from your gutter gratings. Do everything you can to control odors. Plug in an ionizer, burn candles, leave bowls of baking soda everywhere. Ventilate the room in the middle of the night, but otherwise keep it closed. Keep the body under a plastic sheet while it's in the tub.
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.