One of the big items is your attic insulation. Between you attic and your basement's rim joist, it can account for 1/3 of your home's energy loss - which is a big number. You can save thousands over the years with proper insulation.
So what is the right level? Most municipalities require around R-25 now. But if you home is even a few years older, you probably won't have that. But no worries, you can always add to the insulation that you already have.
The ideal amount according to the US Dept. of Energy is around R-45. That number is a little out of reach for most people, but anywhere between R-25 and R-45 is a good number.