When it comes to photography gear, dirt is bad. This is ever so true with dSLR systems. Static electricity causes dirt and fuzz to cling to lens elements, mirrors, and sensors. Each time you change a lens, you open up your camera and invite debris to enter. Small amounts of dust don't typically cause problems, but it can build up until you get unsightly spots on your photos. Here's something to help prevent buildup.
Rocket Air Blaster
The Rocket Blaster is a key tool for preventative maintenance. When squeezed, it directs a focused stream of air at the target, clearing any loose dust or particles. It won't take care of the really stuck-on stuff, but you'd be surprised at what it can do.
I use this tool for clearing off the front and rear elements of my lenses before and after each use. A simple wipe-down may not be effective when static electricity is holding the dust to the elements. I also use the rocket blaster during lens changes to blow out the sensor chamber. This clears the mirror and sensor of any loose dust that may have found it's way in during the lens swap. To do this, just hold the camera face down (so the dust doesn't settle back into the chamber) and blow it out. If you really want to go nuts, you can lock the mirror and get a better shot at the sensor, but it's not always necessary.
Another use I found for the Rocket Blaster is in macro photography. When you take photos of things at high magnification, you make it much easier to see any debris that is stuck to the subject you're trying to photograph — dust, fuzz, pollen, dirt, small bugs, etc. Just take the Rocket Blaster and give the subject a little air shower prior to shooting it, and your macro photos will come out much cleaner.
Physically, the tool isn't a small one. It won't fit in your pocket like a spare battery or lens cleaning cloth will. But if you can make room for it in your camera bag or backpack, it's well worth having it with you. The price is right on this thing, usually between $5 and $10 at Amazon or most other photography shops — certainly much less than the cost of getting your sensor cleaned.