LilyFestre, get some fiberglass batts and tack them up under the floor joists...assuming you can get at the underside of the house. More information here.

Another must-do is pipe insulation for any hot water pipes that run outside, like if your water heater is in a garage, and an insulation blanket for the hot water heater itself (unnecessary, I think, if it is less than 10 years old). These won't keep you any warmer, but they'll save a lot of money.

We just invested in some new double-pane fiberglass windows, to replace the approximately half of our windows that don't seal. They are supposed to be installed later this month, before it starts to get cold here. We also have some good doors on the fireplace so we can enjoy the heat without all of the warm air being sucked into the great outdoors. My project for next weekend is to re-felt the edges of the outside door.