If you are a relative or friend of an expecting mother-to-be, think about giving a membership to a portrait studio as a shower gift.
If the mother-to-be already has a theme for the baby's room (a color scheme, the circus, baby animals, etc.) it's easy to develop a theme for the shower around that theme.
Diaper Cake - Making something that looks like a cake, but is made out of rolled up diapers is a great way to make a beautiful centerpiece that is also a useful gift for the mother-to-be to take home.
When deciding who to invite, ask the mother-to-be to look over the guest list to make sure you don't inadvertently leave someone out.
When choosing beverages to serve, keep the mother-to-be in mind and avoid alcohol and caffeine.