I have flown 2 of the planes mentioned by Jeremy earlier off of grass. The Regal Eagle and the Starfire (mine was a JMP version). I have also operated a Byron F-16 off of grass.

Slightly oversize tires and a positive ground stance are the 2 main things you need, although the Starfire I flew had stock sized tires on it.

Most of Russia's jet fighters were designed to be used off of unimproved (grass) fields. FOD is a big concern with flying jets off of grass, full scale and models.