RPM
If you use spray foam to seal around doors and windows, look for the new stuff from "Great Foam". Comes in a blue can and is guaranteed not to warp your doors or windows. I've sealed around both of my doors with it. Pumped in enough that it came out the other side. Worked great and didn't bother the door at all.
Additional attic insulation will also help. If you check Owens Cornings website they have a link to the DOE that will allow you to type in your zip code, heat type, etc and it will tell you how much insulation you need. There are also many pages of instruction on installing the product--how and where.
A big thing to check for is the rim joists. (aka box rims, end rims, end board-- basically, the upright board that lies ontop of the foundation that the floor joists connect to.) Older homes usually do not have any insulation in this location. The problem is that this is a portion of the exterior wall (especially in 2 story homes), so you wind up with a wall that is say R19 with a 10" strip around it that is only R1. Also, if you are on a crawl space, try insulating the floor. I can give you the recommendations for my locality, but I think the DOE would do you better as you will get the info for your state. If you insulate the foundation it will also help, but be aware that my own testing has proven that pink foam must be covered to below the grade level to prevent roots, mice and bugs from penetrating. Penetration is bad as it can make a channel for water to wick your heat away.)
The good news it that home heating fuel is anticipated to be lower this winter.
SHF