I use simple green and it works good.

1. i wash the tires with water
2. i spray them with simple green (not diluted)
3. spread it around the tire and let them sit for as long as you want
4. use a towel to dry off the excess simple green (tires should be dry and grippy feeling)

i do this between every round at my indoor clay track. It works good and is much cheaper than a "rc" traction compound. The most important thing is to dry the tires. If you dont they are slimy and have less grip than before you cleaned them.