Well, you can check to see if there is a car seat recycling program in your area. There do not seem to be many, but you might find one nearby. You could also contact your a local tech or SafeKids to see if they recycle/dispose of seats.
If those options don't work, then you want to destroy the seat. If you're handy with hammers/tools, you could beat it up yourself.
Otherwise, remove the cover (depending on what it is, you might be able to sell or trade it) and cut the seat straps. Write *DO NOT USE -- EXPIRED!* on the shell with a marker. Wrap the whole thing up in a black plastic trash bag (so that nobody will be able to tell what it is) and set it out with your trash on pick-up day.
