Apply a sealant or caulk around windows, doorframes, sills and joints. On a windy day feel for leaks or use a couple of incense sticks to help identify leaks around windows, electrical outlets, vents and exterior doors. As well look for spider webs – if there is a web there is a draft.
Use plastic window covers to help prevent heat loss.
Keep return air grills and heating vents clear of furniture, rugs and drapes, so there is no interference with the flow of heat through your home.