Going to keep this one short and sweet. At various street festivals around Worcester MA I always see lots of people with DSLR's. I always, always want to go and correct every one of them in some manner or other.So here's some basic things you should know. Don't change your lens with your body facing out into the air, treat it like an open wound while in-between lenses. Change your lenses as quickly as possible to minimize dust and debris floating in. Dust on your sensor will show up in every single picture you take.
Don't put the camera around your neck, loop it under your arm. Having it under your arm and at your side allows you to protect it a bit better. No longer will your camera swing into random things everytime you turn your body. Black Rapid makes an excellent camera strap called the R Strap and it allows you to leave your camera hanging down at your side at all times.
Pop up flash, don't do it! Shooting someone with flash straight on ruins the picture, think about a deer in headlights; no ones happy. Pop up flash is okay for crime scene photos or documenting a grizzly murder but when it comes to your photos you don't want anything to do with it. Consider hopping on amazon or ebay and finding a flash gun for your DSLR. Basically a flash that slides into your hot shot on your camera and allows you to bounce the flash off a ceiling or wall or other to create something far more pleasing. There are also many adapters for your flash gun such as reflectors or Gary Fond light sphere or a mini softbox. You can also use a cord or pocket wizard to bring the flash away from the camera long enough to get the right shot.
Take off the lens cap. You can scrap the front element all you want and it won't effect your pictures, just don't mess with the rear element. If you make one little scratch in your real element, kiss good pictures goodbye. Lens caps should only be used for storage. Don't be lame.