Jackson West, writing for SF Appeal, has a great article about the best way to get your stuff from A to B on two wheels:
Nothing says "post-adolescent refusal of adult maturity" like a front-mounted bike basket to go with your ironic rainbow streamers and twee bell. Kidding! A basket is actually a great way to haul the occasional boxy load, with even the cheap steel-mesh cage types weighing only a few ounces and therefore no drag on everyday riding, though they can make maneuvering up tight stairwells a little more awkward. They do take a modicum of tools and know-how to install, both of which any good bike shop will be happy to help you with.
Better yet are folding baskets. The first thing I want to add to my 18-speed cruiser is a rear rack with a pair of folding, side-mounted baskets for trips to the farmer's market. They weigh the same, folded or not, but when empty make the bike no less manageable when walked or carried.