I have found that the best centerpieces involve candles and flowers. The two seem to go so well together. Silk flowers or real flowers can be used and candle bowls with floating candles, or glass stemware with votive or tealite candles also make great centerpieces. The bowls or stemware are reusable so it makes it very economical.
There are lots of variety out there and http://www.candle-boutique.com has tons of them.