Congratulations on taking a step toward better image quality and purchasing a DSLR. Now that you've got that big sensor and big camera, don't expect that your images will instantly be leaps and bounds better than a point-and-shoot. This is a totally different league. Buying into the system will make your camera less and less future-proof and learning how to take better images is the only thing that will really set you apart from a point-and-shoot user.
Great images are made - besides the right gear they require the correct lighting, attractive composition, unique perspective and great subject content. Part of all this is keeping in mind what makes your subject unique and could also require playing with the subject and camera a bit. Here are some essential upgrades for your DSLR.
Your First Prime Lens
The purchase of your first prime lens is one that will give you memory cards and memory cards full of great image creating potential. For the uninformed, a prime lens has a fixed focal length. For example, a 50mm lens or an 85mm lens. There is no zoom range of any sort.
You're probably asking yourself, "Why the heck would I want that?" Because even though you only get a fixed focal length, this lens is bound to be the sharpest in your camera bag for a little while until you make the purchase of a more expensive and better quality lens. Besides this, the aperture is likely to be much larger, which means that you'll be able to take images with a shallower depth of field and in lower light without having to raise your ISO sensitivity too much.
With the addition of a prime lens to your kit, expect to capture some breathtaking night photos as well other subjects. Though the purchase of your first prime lens will allow for more creativity than a zoom, it should be noted that new users usually get caught in what is called, "The Bokeh Trap." This is when users shoot images with their aperture wide open all the time and end up making silly mistakes, like losing very important and essential details in their photos.
Most of these lenses are around the $100 range or a bit more. I personally call the purchase of your first prime lens the first step towards liberating your photography because of the potential of some of these little pieces of glass surrounded in plastic.
An External Flash/Diffuser
The problem with lots of on-camera flashes is that their light tends to be very harsh and not very flattering at all. Getting something like a Gary Fong Puffer and putting it into the hot shoe can alleviate these problems, but even then it takes practice.
The best alternative is to purchase an external flash with a tiltable and rotatable head that could perhaps act as a slave while your camera acts as a master. In layman's terms, this means that your camera will be able to wirelessly operate your flash. For example, my Canon 7D can trigger my 430 EX II to fire when the 430 EX II is set to slave mode. In order to do this, I need to raise the pop-up flash on my 7D and set it to trigger the 430 EX II. Most of the time when I'm shooting products, this is what I want. Depending on the situation, I may want the on-camera flash to actually fire and be part of the photo.
An external camera flash will give your photos beautiful light as well as enhancing the auto focus performance of your DSLR. As a tip, most photographers tend to just point it towards the ceiling or bounce the light off of something else like a wall, umbrella, etc.
While this may not seem like much, lighting is perhaps the most important variable in photography. Excellent photos can be achieved with cheap, good quality lenses and excellent lighting.
A Neutral Density Filter
This is probably the item that will appeal most to landscape shooters. A neutral density filter is a filter that goes over your lens and allows your images to be darker, brighter or have different looks such as a warmer or cooler look. An ideal neutral density filter reduces and/or modifies intensity of all wavelengths or colors of light equally, giving no changes in hue of color rendition. They're unusual in that they work by spinning them around until the colors change. Landscape photographers make the most use out of them because they enhance the color range of their camera because they allow greater versatility in the aperture changes and exposure time.
This isn't recommended for everyone, but they are fun to play with if you want to get your feet wet on the beach to capture a gorgeous sunset.
Gorillapods are tripods with flexible legs. The flexibility allows for easier positioning of the camera at certain angles. Additionally, a camera can be wrapped around a pole or other surface this way.
I personally ditched mine for a long time because of the fact that I shoot with prime lenses or image stabilized lenses until I found another great use for mine. Gorillapods are excellent for wireless flash or light setups because of their ability to be placed nearly anywhere. Whenever I do a shoot that requires use of my wireless strobes, I always bring gorillapods with me because of the fact that I really don't like large tripods or stands for lights. Granted, they are inescapable when I need to fire into umbrellas, but gorillapods can still tackle most tasks.
A New Strap
This won't be necessary for everyone, but the parent that is shooting their child's soccer game or playoffs for football season will perhaps take great value in having a faster and more versatile camera strap. Great examples of these are the Black Rapid R-Strap or the Sun-Sniper camera strap.
These straps work different by screwing into the tripod mount of your camera and then allowing it to hang freely at your hip. In turn, the photographer has faster access to their camera when they need to auto focus in order to get the shot.
If you do perhaps get a longer zoom lens with a tripod mount on it, you can always screw the strap onto the lens instead.
If there is anything I wish people did more often, it would be reading their camera's manual. Going to the index of your manual and searching for "wireless flash" is a heck of a lot simpler and less of a hassle than getting on the phone with a customer service rep of some sort or going through your cameras menus to have to find something yourself.
The manual was written to help you be able to make the most of your camera and perhaps it will even help you to realize that the camera is a lot more powerful than you previously thought.
The next time you have a question about your camera, try to rely on yourself and read the manual instead. Otherwise Googling, "How to switch my camera into aperture priority" could become commonplace for you.
An Extra USB Cord
Not every computer has an SD or CF card slot. The best bet is to just get another USB cable of some sort and just connect the camera to the computer. Be sure that the computer can read and process the RAW files from your camera.
A Photography Instruction Book, Course or DVD
Getting an education in how to create and shoot better photos is perhaps the biggest upgrade that you can do for yourself. The learned information is always invaluable. Courses encourage students to go out there, make mistakes, and learn from them. Experience is the best teacher, but guidance of some sort can always help.