at the same asian market that i got my galangal i bought a small tray of shiso leaves.
any ideas how to store them or preserve the for future use
I've had reasonable luck just keeping them in the refrigerator, but if you want to keep them longer, one traditional way is to salt them. Sprinkle some salt between each layer and keep them in a non0reactive container in the refrigerator. As you might guess, they'll exude some water and turn a darker color. But they remain tasty. Rinse or not, depending on the taste you're going for.
Another option is to freeze them. Like with the salting method, they turn dark and the texture changes but you can still use them.
There are ways to preserve shiso leaves with soy sauce, etc. but then it because a 'pickle' or something to put on rice. If you are thinking to use as a shiso leaf, like next to sashimi, it must be fresh. Sorry, but best to buy a day or two before using.