Camera bags are great — they allow you to carry all your important equipment around while you're out shooting. But as great as they are, sometimes it's nice to leave them behind and free yourself up a bit. Then what do you do with those bare-minimum pieces of equipment that you absolutely must have with you?
Put 'em in your pocket. For me, the bare minimums (in addition to the camera and lens) are a spare battery, spare memory card, and a lens cloth. These are all pocket sized items and can easily be carried without a bag. But rather than tossing them in your pocket to rattle around with your keys or cell phone, why not package them together in their own little unit?
Doing this “packaging” allows you to keep everything together. It also allows you to toss the unit in your bigger camera bag one day, then pull it out and stick it in your pocket the next day. I'm able to fit my battery inside of my memory card pouch, and my lens cloth has a clip that I attach to the keyring on the pouch. These three things ALWAYS stay together and they ALWAYS travel with me when I have my camera.
What are your essential items? And how do you pack them around?