You can have the energy company suggest things to you. They might even come and perform an energy audit of your house. Some inspectors will set up a pressure system in the house where they close all windows except one and then hitch a high powered fan to suck air into the house. Then they use what amounts to an incense stick around recessed lights and windows to look for air leaks.
At 15yrs, yr furnace is probably ripe for replacement. There are high-efficiency models now that could cut yr costs by as much as 10-15%/mo. Keep in mind, though, that this savings might be more in the winter and less in the summer. Overall, you have to weigh the cost savings vs. the cost to replace. I'd bet the payback is 5-7 years.
If you want to check for leaks yrself, start in your attic. Look for wrinkled insulation and gaps. Peel it back and look for leaks around ceiling lights. Caulk appropriately. Check that yr fixtures are IC rated if yr insulation is going to touch it.
If you have a fireplace, make sure the damper is closed when not in use.
You might consider a storm door if you get decent wind directly up against the front door. Again, that's a few hundred $$ initial cost vs. $10ish savings/mo.
An expensive but very effective thing is to replace windows with double or triple pane windows. At the time or replacement, you can also double check that the jambs and casing are properly insulated and caulked.
Curtains or blinds on windows can greatly reduce drafts and heat loss at windows.
Outlets on exterior walls can also be a source of small air leaks. There are foam inserts that you can buy to improve the insulation in these if it hasn't been done already.
If it were me, I'd probably ride out the furnace until it conks; the technology's only getting better each year.