It might be a good time to invest in a pin vice if you don't have one:
http://www.ebay.co.uk/itm/3x2mm-Magnets-Pin-Vice-3x-3mm-Drill-Bits-Airfix-Warhammer-40k-Scale-Models-/152092780516?hash=item23696fa7e4:g:A48AAOxyni9TAgTd
You can drill a small hole in the banner, push a toothpick into the hole and then fix it to your model. You can do that by making another, slightly bigger, hole in the hand of the model and pushing the toothpick through. Hey presto, banner. Don't forget to superglue it as well!
Be careful when drilling that you don't accidentally go off target and ruin the hand or the banner. Practice on some leftover sprue first if you haven't done it before.