How can I increase the traction of my robot's wheels?

My school has a robot team that is new this year and our experience with robot wheels has been less than favorable. We originally had wheels that worked, but we had to make a change that rendered them too small. Now we need 5 inch wheels but it is impossible to find wheels that will work for us. Finding wheels that are 5 inches alone is nearly impossible let alone wheels with the right bore size and width. We came to the conclusion that we had to make our own, so that is exactly what we did. The only problem is that we made them out of hard plastic because we need to stay low on weight. I was wondering if anyone has any ideas of what we can do to our wheels to increase their traction because they get very little and it causes our robot to slide around. We have tried electrical tape with not so phenomenal results and we had used a rubber-type dip on the old wheels that kind of worked. Any suggestions would be appreciated.