Some really easy things you can do: Spray paint small parts (gussets, wheel hubs, etc) a highlight color like
this. Spraypaint doesn't look as nice as powdercoating, but it will hold up decently well if it's not in a super hostile environment (read: getting hit all the time). It also takes a lot less time, and doesn't require you to find a powdercoating sponsor or pay for it.
As mentioned above painted lexan panels, like
this and
this also tend to look nice, and save you from having to wait to assemble the robot while it's being painted or powdercoated.
Or just a clean robot with some nice
vinyl decals* can look pretty good.
As the other posters mentioned, powdercoating looks really good, as does anodizing aluminum parts, but it can be time consuming, and often requires pre-planning to make sure you can get it done in time before you assemble the robot. The biggest thing you can do is make your robot look clean. Tie down and rout your wires well. Make sure stuff is filed and looks well made. Add a little color and you'll be looking sharp!
*Disclaimer. I think those are vinyl decals on 67's 2009 robot, but they could just be painted on. The effect is the same though.