Essential accessories for a digital SLR camera

• Camera bag/backpack -This is the toughest item to shop for. Professional photographers have many bags for different situations. A discussion of camera bags deserves its own post.
  
• Manual - Always bring your manual. You never know when you need to look up something. Keep your manual in a good ziplock bag, so it doesn't get wet.
  
• UV filter - To protect your lens in rough environments
  
• Lens cleaning kit - A kit typically includes a blower brush, lens tissues, and a bottle of lens cleaning fluid. I also like getting packs of lens wipes for convenience. A lens wipe is a lens tissue already coated with lens cleaning fluid and sealed inside a packet. Zeiss makes some nice lens wipes.
  
• Waterproof pack cover - So the camera inside your pack doesn't get wet. For example, the Sea-to-Summit pack covers.
  
• Extra battery - Never go without a backup battery. This is also useful for cold weather situations. You can keep the backup battery near your warm body and swap it with the battery inside your camera. Keep swapping back and forth whenever the battery in the camera gets too cold to operate.
  
• Extra memory cards - I recommend 4 GB for 10-12 megapixel cameras and 8 GB for higher megapixel cameras. The speed of the cards should match the transfer speed of your camera. Cameras with higher frame rate will need faster cards. The idea is that you spread out the risk. Instead of getting one huge memory card (say 16 or 32 GB), you get 3 medium sized (say 4 or 8 GB) memory cards. If you lose one or it gets corrupted, you're still OK. You only lost the images on that one card.
  
  
• Backup camera - If something goes wrong with your main dSLR camera, you need a backup. In fact, this is what separates a pro photographer from someone who just shoots as a hobby. A pro photographer has to have backup equipment because they can't tell their client that they took zero photos due to equipment problems. The backup camera doesn't have to be another dSLR. You can use a point-and-shoot camera. A good thing about point-and-shoot cameras is that they are great for macro because of their inherently short focal lengths. It's difficult to shoot macro on a dSLR without a specialized macro lens.
  
• (optional but important) Tripod/monopod - If you want to do any macro or long-exposure work, you will need a tripod. You can get by without a tripod if you focus on event shooting, but eventually you will want a tripod. Monopods are useful when tripods aren't allowed or if you need some freedom for movement.
  
• (optional but important) Flash - Important for shooting inside dark rooms like at a party. You want a flash that can be tilted at angles. This is useful for bouncing the flash off walls and ceilngs. You can use the built-in flash on your camera, but it's not very good. A small flash is fine. Examples are the Nikon SB-400 Speedlight and the Canon Speedlite 270EX. If you have the money and don't mind the weight, you could get a big, pro-level flash.
  
• (optional but important) Better camera strap - I didn't like the strap that came with my camera so I got the Op-tech Pro strap. It was much more comfortable for carrying heavy lenses around my neck.