Is there a way I can improve the sealing around my windows to increase their efficiency?

There are several answers, depending on how they replaced the windows. Since it is in a historic district, I assume they are wood windows. Did they install sash replacement kits or remove and replace the entire window and jamb?

If they installed new windows -- the sash is probably pretty tight to the jamb-liner and there is probably a large gap between the window jamb and the rough framing. You probably have air leaking in around/through the casing. With a blower door and an infrared camera, you can pinpoint where the problem is. You may be able to caulk around the casing, stool, and apron and seal up most of the leaks. If this does not work, you may need to remove the casing and seal around the window jamb with a spray foam.

Typically, in older homes, sash kits are used to replace windows and the existing windows are often out of square. When the new sashes are installed, there are a lot of gaps around the window. If that is the case here, it can be solved by installing some interior storm windows. There are many professionals who make these, and some specialize in historic windows -- they even have low-e storms. Stormwindows.com is one resource, although there are plenty out there. A lot of older windows are built with window weights and there are large gaps between the jambs and the rough framing as discussed above. If that is the case, you need to follow the directions for the new windows as well.

Remember, for an energy efficient home, it is typically better to seal up the holes (air seal) in the ceiling/attic and floor/crawl space before focusing on the exterior walls so be sure to check these areas as well.

Good Luck.