To help reduce air leakage, sealing the areas where cables are run down into the walls from unconditioned areas (like the top plates of walls in the attic) is the best option, but often times those areas are in less than accessible spaces, or spaces that most folks wouldn't want to access at all, even if they could. So, often the easiest way to reduce air leakage is by focusing your attention to sealing the inside walls.
While gasketed wall plate covers are available, there is a cheaper, easier way to help seal up these holes in our walls. The same compressible weatherstripping with adhesive on one side that your hardware store sells to help seal windows and doors can be used to help seal these covers, by creating a seal around the edge of the cover, like seen below:
You can also see the childproofing outlet caps on the electrical outlet next to the pictured data jack, which not only prevent inquisitive children from re-styling their hair (or worse), but also helps to reduce air leakage.
It might seem like sealing outlets and data wall plates would result in a trivial difference; and the reduction in leakage will be fairly small, but sealing all your wall plates like this will make a difference, especially in homes that have already sealed up any large leakage areas.. When weatherizing, sometimes it is best to 'sweat the small stuff', or you will end up shivering!