A few readers have written about digging up geraniums and storing them indoors over the winter. If you plant a lot of geraniums every year, this can save you a lot of money, but if you've just got two or three in pots by the front door, you might not want to bother.
You can simply bring the pots inside and treat them as houseplants, but they'll likely grow leggy and sickly, so this isn't your best option.
Instead, before the first frost, dig them up, shake the loose soil off and place each plant into a paper bag. Take them out once a month over the winter and soak the roots in water for an hour or two, then place them back into their bags.
In late March, cut the plants back, removing any dead or limp stems, and pot them up. Keep them by a sunny window and resume watering until May, when you can bring them outdoors.