Floating Flower Tip - Most flowers float pretty well on their own until they start to pool with water and slowly sink. For a longer lasting centerpiece, use bubble wrap to make little skirts for your flowers.
Tutorial: DIY Tissue Paper Pom-Poms this tutorial is pretty good. Use square sheets or tissue for a rounder Pom Pom. Tie the middle with ribbon and tied several on ribbon and hung them for a backdrop. Use thin tissue paper and fluff gently.