Samhain is the time of year when many Pagans and Wiccans celebrate the cycle of life and death. This Sabbat is about the end of the harvest, the calling of the spirits, and the changing aspects of the god and goddess. Try some or even all of these ideas -- obviously, space may be a limiting factor for some, but use what calls to you most.
Colors of the Season:
The leaves have fallen, and most are on the ground. This is a time when the earth is going dark, so reflect the colors of late autumn in your altar decorations. Use rich, deep colors like purples, burgundies and black, as well as harvest shades like gold and orange. Cover your altar with dark cloths, welcoming the coming darker nights. Add candles in deep, rich colors, or consider adding an ethereal contrasting touch with white or silver.
Symbols of Death:
Samhain is the time of the dying of the crops and of life itself. Add skulls, skeletons, grave rubbings or ghosts to your altar. Death itself is often portrayed carrying a sickle or scythe, so if you've got one of those handy, you can display that on your altar as well.
Some people choose to add representations of their ancestors to their Samhain altar - you can certainly do this, or you can create a separate ancestor shrine.
The Harvest Ends:
In addition to symbols of death, cover your Samhain altar with the products of your final harvest. Add a basket of apples, pumpkins, squash, or root vegetables. Fill a cornucopia and add it to your table.