Handbags are often a microcosm of one's life in general. The same rules of organization still apply:
Keep like things together. Use small zippered bags to contain such items as makeup. This makes it easier to move items when changing handbags, as well.
Give everything a home. Choose a handbag with pockets and designate one for sunglasses, one for keys, and one for your cell phone. Assign a place in your wallet to keep receipts, another for store credits and gift certificates.
Put things back. Rather than haphazardly tossing things into your handbag, take an extra minute to put cash or change in your wallet, receipts in their designated place, or a colleague's business card in your planner or notebook. Aim to have nothing floating at the bottom of your bag except larger items like your wallet, eyeglass case, or planner.
Flickr photo by Dvortygirl
For more cleaning tips, check out What's A Disorganized Person To Do?