If you're buying mint at the market, look for bright green, crisp leaves with no signs of wilting. Place the stems in a glass containing a couple inches of water, and cover the leaves loosely with plastic wrap or a zip-top plastic bag (do not seal bag). Refrigerate up to one week, changing the water every other day. Fresh mint can also be frozen for later use. Simply rinse the leaves, pat them dry, and freeze them in a zip-top bag (the leaves will darken once they're frozen, but that won't affect the flavor). Later, pull out what you need, and return the rest to the freezer.