You can buy electrical outlet insulation and use these on all of the outlets on exterior walls of your home. Using an old rug or towel to fill/close any gap at the bottom of entry doors will help keep cold out and heat in. Make sure fireplace flues are closed and sealed properly when not in use.
Don't run bathroom fans for longer than 15 minutes, they increase the amount of outside air being brought in. If your shower doesn't dry, use a squeegie to get the water into the drain and avoid problems with humidity as well.
There are actually insulated quilt shades for windows and you can take a quicker and less expensive approach. You can create your own window quilts if you have drafty windows, sewing isn't required. Just hang a quilt or blanket on a sturdy curtain rod against the window. For it to look best and not clash with your decor (or be seen from outside) you may wish to sandwich the quilt between some white sheers facing the street and your curtain or put the quilt between two curtains.
Depending on the actual outdoor temps and the amount of sun your home can take in during the day, it may not do you any good to open your curtains during the day to let the sun warm your home. Be sure to take this into account as many homes don't have the most efficient windows and are a source of heat loss in winter.
Make sure your heating system gets a tune-up every year. Most Americans don't do this and they let the system slowly fall into disrepair. Some fairly minor issues when overlooked CAN affect the amount of heat getting into your home from your system, while your doors, windows, and even electrical outlets will happily let it out.