Okay a nice handful of straw will slow down the rotting, as the moisture from the ground will slowly seep through, but just keep changing it say twice a week if that and you will be fine, or a more efficient way if to stick a small square of polystyrene or however you spell it under, and that does the job.
The key is to just raise the fruits a tiny bit of the ground so they dont rot, anything will do so long as it isn't a quick absorber.
Hope this helped,
Joe