I think air infiltration doesn't get the attention it should. Air sealing gives the best heating savings for your dollar when compared to almost every other building envelope improvement; insulating exposed concrete basement walls is the only other thing I can think of that comes close.
I'm pleased to see marketing funds being spent on this issue, like this commercial featuring David Suzuki.
Using Hot2000 to model a house located in Halifax, NS with R26 walls, R40 ceiling insulation, and energy efficient windows, about 27% of the heat loss in January comes from air infiltration. Hot2000 doesn't count the heat loss due to the moisture in the inside air (latent heat), and recent research indicates it underestimates the total air infiltration rate. Considering Hot2000 errors, a more accurate estimate would be 40% of the heat loss coming from air infiltration.
Although I think the best way to find air infiltration points in a house is with a blower door test (typically $50-$100), there is a cheaper way. I read on builditsolar.com the idea to turn on a clothes dryer, then go around your house checking for leakage. If you have a kitchen range hood exhaust fan and bathroom exhaust fans, turn these on too. In addition to checking the usual places like around windows and electrical boxes, check for air infiltration along the floor on outside walls. If you have tile or hardwood floors, a bead of translucent or clear caulking between the baseboard and floor can cut down air infiltration.