To get the job done, you need the right tool -- you can't assemble IKEA without that IKEA tool, and if you see thugs stealing a purse, and you're Australian, you're doomed to failure without a late-80s limousine TV antenna. Choose the right bag, from Gravis's spring/summer line, only at Spare Parts.
Working closely with their motley cadre of advisers (snowboarders, surfers, musicians, artists, the occasional lawyer), Gravis has just dropped a fresh batch of bags optimized for various pursuits, but all constructed for comfort (ergonomic padded straps/backstays), durability (uber-strong nylon, waterproof interior...), and style (sexy "spicy orange" vinyl; uber-strong purple nylon). Backpack-wise, there's the sizable but tightly constructed "Metro" w/ neoprene laptop cradle and elongated exterior pockets, perfect for digital-age homeless people; the medium-sized "Hubble", with hideaway straps for skateboards; and the compact "Sure Shot" with hidden side "stash pocket" -- which'd probably come in handy for Hubble owners. As for messengers: for folks who carry their whole office there's the "Digi", with three zippered interior compartments and a back sleeve attachable to rolling luggage; people who're constantly losing stuff'll dig the "Hobo"'s zippered main compartment and "easy-access" under-flap pocket; and people with tiny shoulders'll love the pint-sized "Sidearm", with iPod holder and easy-access mesh pocket for writing utensils, just in case you find yourself transported to "the past".
If you have very little to carry, grab the "Mantis": about the size of a fannypack, it's an over-the-shoulder job that, when snatched, will have you beaten, bruised, and cursing advances in limo entertainment.