On tall grass, it actually helps to make the nosewheel larger. I'm using a 2.75 on the nose, and 2.5's on the mains on my beater-trainer. On pavement, this would be a no-no, but on grass, having the plane sit slightly nose-up gets it lighter on the wheels faster, and up and out that much sooner. In the past, I've used as much as a 3.5 on the nose of a .40 size trainer, and 3 or 3.25 on the mains to get off really tall grass.

I totally agree about getting the light wheels, like the foam ones. Heavy wheels are a pain. Also, go to a lower pitch prop. I'm using an APC 10x5 on my trainer, and it helps pull though the grass to get moving.