I suppose you could try adding banana chips to your mash, haven't tried myself though so can't really offer any valid comments, but if you really want a banana flavour coming through in your beer maybe it'd be an idea to use one of the wheat beer yeasts, like WB-06. Ferment at around 72 to 74F for the first couple of days of fermentation and then ramp up ferm temps a bit more from there onwards. That should get you some noticeable banana esters happening.