The best way to do that is with a hard case like the new CandyShell line from Speck. The new line of cases are available in five different two-tone color combos. The outside of the case is hard, but the inside is soft. Nobody wants a case that is going to scratch their phone.
The new colors include BubbleMaker Pink, BatWing Black, AquaBerry Blue, WaxStick White, and JamJar Purple. The interior of the case is rubberized and the case has openings for all the ports on the iPhone. The case also comes with a cleaning cloth and a screen protector. The case sells for $34.95 and the purple/pink version is only available at AT&T stores.