It seems like everyone has trouble with banana stains. Here's what I do. If the stain is fresh (i.e. the baby just ate), then I rinse off any chunks (in hot water), and then I put liquid Era on it and scrub the stain with a toothbrush. Periodically, I rinse where I scrub with hot water to see how much of the stain comes out. I do this until the stain is completely gone or almost gone, and then I'll rinse out all the Era (if you don't, it might fade the colors) and throw it in the wash with other clothes (in cold water). I always do a second (cold) rinse in the machine after the initial wash.
If the stain is older, then I'll put Era on it and let it sit for 5-10 minutes, then I do the same as the fresh stain.
I use this method for all my stains, and it seems to work really well.