Take ten minutes and walk around your house to make sure all air vents and air returns are free of obstructions.  Not only does this make your HVAC system run more efficiently, it will make sure that the air you are paying to heat/cool actually makes it to the place you need it.  

Removing obstructions is especially important when you have floor vents like me.  Over time, furniture, drapes, and other items can block the flow of air.  If you absolutely must have a piece of furniture or other item directly on top of a floor vent, buy an inexpensive air deflector to place over the vent so it directs air into the room.

Another tip — if you have floor vents, pull up the vent covers once a year or so.  In a previous home, I found 3 - 4 inches of construction debris left over from before I owned the house.  Apparently, when the flooring contractor was installing new linoleum, they just swept the trash into the nearest hole in the floor!  That meant the HVAC system was pushing dust and other junk into the air!

Photo credit:  budadams