Microfiber cloths are the greatest invention. The suck up oils and dirt though capillary action without the need for cleaners in most cases. For stubborn skin oils inside the crevices you can use a few drops of lens cleaning solution, nothing else. You can find micro fiber cloths at retailers such as Radio Shack and Wal Mart. Avoid ones that are for car cleaning as they may have rougher fibers and are not true microfiber cloths. Make sure you have clean hands and begin rubbing the microfiber cloth around all the buttons, from the center of the button outward, not forcing dirt deeper into the tiny crack.
Step2
Wipe around the SLR camera's lens mount moving from the inside toward the outside. Remove the compact flash or SD card and wipe it down as well as the insides of the door. Don't attempt to remove the lens and clean your camera's sensor without consulting your owners manual or reputable camera shop.
Step3
Don't forget about the flash. Dirt under the flip down flash will cause scratches if not removed. Install a screen protector on the LCD display, this will help preserve your camera's value and keep the display clear. For rough use there are products such as Camera Armour that offer a silicone glove custom made to fit many camera models such as the Canon 40D.
Tips & Warnings
Use preventative maintenance and keep your digital SLR camera in a good sealed case and use skylight filters or lens caps on the lens.