I watched a video on window boxes on the HGTV website yesterday. You might find that helpful, it was only about 5 minutes. One of the window boxes that was displayed kept all the plants in their pots. It was a well done piece.

Hi there:
If your window boxes are made of wood, it is better to leave your plants in thier pots. I have found if your place soil directly in the window boxes, they will start to rot, and the soil when wet becomes rather heavy too. One od mine came loose from my shed, and had to be repaired. I just leave the plants in pl;astic pots and lift them out to water then replace them. I tried Impatiens ,but they became so large they blocked the window. Pansies are good, and dwarf Marigolds. Dianthus is neat because it trails down the sides. I like to change mine with the seasons. Petunias
are great for the Summer.I have Charmed Velvet Oxalis in mine now. It has a dark almost Black leave. I am going to put some Orange Panzies in for Halloween.

For winter I was going to try some small cabbage or kale, asters, pansies....

If you google around you'll find some ideas, too!