We use the plastic registration boards for the numbers (available at almost any marine store). I have them attached to the line that runs from the bow down the side of the boat with reusable zip ties (the kind with the little levers so they can be removed and reused). However, I've found that they can even remain attached when the boat is deflated and folded and they don't get in the way. The other option is to use lettering that is specially made for inflatbles, but I've noticed that even those start to fall off in short time; and the registration stickers never stay on more than a few days.
This is one of those areas that drives me crazy. We registered our dink just like you are supposed to but the damn reg stickers and numbers don't stay on for anything especially after deflating and inflating. Finally we just said screw it and wrote the numbers on with a big black marker and the one remaining reg sticker (other when fell off for good) we just stuck on the wooden transom. I know it is technically not legal but comon what am I supposed to do? It is a damn blow up dink afterall.