Aside from my camera and lenses, my most used piece of equipment is a micro fiber cloth by Nikon. A clean lens is essential to producing consistent quality images, and there are a number of ways to accomplish this. Obviously, the best way to clean a lens element is to wet clean it with the proper chemicals and wipes, but that's hardly feasible when you're out and about.
Micro Fiber Cleaning Cloth
This handy little tool is small enough to take everywhere with you, but big enough to clean the largest of lenses. The cloth is made of a very soft micro fiber material, and it's about 5 by 5 inches in size. It fits into a small elastic pouch with a clip attachment, and the overall size is smaller than a 9 volt battery.
I typically use this cloth while I'm out shooting to give the front lens element a quick wipe to ensure I don't have any stray dust or fingerprints mucking up my shots. You can also use it to clean the LCD screen, but I tend to use my shirt for that so I don't get skin oils on the micro fiber cloth. I use the cloth for more than just my SLR gear; it works great for cleaning up the point & shoot lenses.
My cloth stays in my camera gear backpack with most of my other stuff, but I'll take it out and stick it in my pocket along with my spare battery if I'm packing light. You can also clip it to your neckstrap and just leave it there so you always have it handy. And if you're a real photo geek, you'll put this baby on your keys as a keychain.
Overall, it's a good compact tool for light lens cleaning. It doesn't work great for really grimey situations, but it will take care of dust and fingerprints no problem. At around $5 to $10 at Amazon or any photo store, it's hardly worth not buying one next time you make an order.