I planted my north facing window boxes with the blue and white combination of leadwort with white winter pansies. Looks fabulous and as the weather gets colder the leadwort stems and foliage will gradually turn red.

Just cut branches of evergreens, or buy them. Include holly berry branches, especially the Ilex verticillata with its deciduous leaves. Some blue spuce, Port Orford Cedar and end it with a big bow that you can remove after Christmas. Think of it as a big flower arrangement and just place the stems directly in the soil. You can even include live pansies at the bottom. This display should remain nice most of the winter. Add bulbs for a real punch in spring!

You've probably already figured out a solution for your windowboxes by now, but I second scenery's suggestion. I recently did this with 2 large containers that flank our front steps and they look fabulous (if I do say so myself :)! I left the dusty miller in there from the summer display for contrast (it was still looking fresh) and filled them w/a combination of blue juniper tips, unidentified spruce? branches w/attached pinecones dangling over the edge, blueberry juniper branches, unidentified branches w/gold tips, white pine branches and red-twig dogwood stems. Before placing in the pots I made fresh cuts on everything, soaked in water for a couple of hrs. and stuck into moist topsoil. Best of all, everything (w/the exception of blue juniper tips which I purchased at a local nursery) was from my parent's yard so the cost was very minimal. I'm hoping they will stay fresh until spring!

My most beautiful winter boxes ever contained ornamental cabbage, anise, pansies and snapdragons. This was a couple of years ago and the rest of the selection escapes me. I think the success of this particular attempt was due to choosing lots of different varieties with a wide range of textures, heights and colors, then literally cramming as many healthy plants in that box as I could. Wow, it was a thing of beauty for months!