How do you prepare Jello with fruit pieces?
The only trick I know is to let it start to firm up before mixing in the fruit.  You can pour a layer, let it slightly firm up (it'll be kinda a thick goo), add the fruit and the rest of the mix.

... but you don't want to let it set up completely; then you'll just have two layers that haven't bonded well with fruit stuck in there.

Exact time for it to gel depends on what temperature you're resting it, and the size of the container you're chilling it in (mostly it's a matter of surface to mass ratio)