Navigating the vast expanse of DSLR equipment and accessories can be a daunting task. There is so much available out there, especially when you are using a camera from one of the major brands, particularly Canon and Nikon. Hopefully, with this article I can help you build a good, relatively inexpensive kit that can cover 95% of the shooting situations an average amateur weekend photographer will be presented with.
To begin, lets talk about lenses. These are the most important pieces that you will buy for the camera. Nothing more directly affects the quality of the picture and the flexibility of the camera more than the lens. Most DSLR's are purchased with a "kit" lens, the lens that was included with the package. You may even read around the Internet that the "kit" lens is usually garbage. I disagree with that assessment. These manufacturers know what they're doing. Even more so, they know their consumers. Be happy with the kit lens. It's your friend. It's usually quite versatile, covering a decent focal range. When you go to purchase accessory lenses, think about what you want to shoot with it. Are you shooting landscapes? Then you most likely want a wide-angle lens. Do you plan on taking nature shots of easily-spooked animals? You probably want a long telephoto zoom lens. If you want a do-it-all lens that can handle most situations, a "walkaround" lens, then you're probably looking at a relatively fast (low f-stop) lens, with a decent focal length. Image stabilization is a plus here. I carry my kit lens (18-55mm), a super-telephoto zoom (75-300mm), and my "walkaround", a 28-135mm Image Stabilized lens, which almost never is replaced with the other lenses.
As far as used and off-brand lenses, buy them with confidence. I have, and still use them. Just do a little research beforehand, and in the case of off-brand lenses, stick to the major manufacturers, like Sigma and Tamron. You can realize very large savings, as long as you make sure the lens you purchase is compatible with your camera. This holds true with flash units as well.
For your lenses, you'll want a skylight filter for each of them. This is a must. Your lens is the most exposed piece of your camera, and a lens with scratched glass is useless. A filter can help protect the lens, and replacing a filter is much cheaper. I use skylight filters as they're usually the cheapest. Other filters you may want to look at are a polarizer and neutral density filter. A polarizer is an extremely fun thing to play/shoot with. You can create a richer blue in skies, subtract reflections from windows, or add reflections to water. A neutral density filter is used primarily outdoors, for the sake of using a slower shutter speed in brightly lit areas.
You'll probably need a memory card. Buy big. As big as you can afford. Also, don't buy in a store. Buy from an on-line retailer, such as http://www.newegg.com. You will save a lot of money this way. More memory means more shots. If you're enjoying the experience and having fun learning about what composes a good picture, you'll want to shoot many pictures. More space is always beneficial. To support the memory card, if your computer doesn't already have one, get a memory card reader. They're cheap, and much more convenient than plugging your camera into the USB port. There are a ton of memory cards out there, with different writing/reading speeds and capacities. On the whole, I would put the priority on size over speed. If you can buy both fast and big, then by all means, a fast card is nice to work with, and will generally allow more shots in a shorter amount of time.
External flash heads are nice to have, but are situational. In my experience, I use mine very sparingly. Very seldom am I taking shots inside that are so poorly lit as to require a flash, and in those situations the built-in flash can perform acceptably. If you envision yourself shooting many indoor shots with poor lighting, then an external flash is a necessity. Again, buy used or off-brand here, making sure it is compatible with your camera's monitoring system. A bounce and swivel head is usually pretty standard on these lens, and for good reason. You can move the head of the flash around to prevent as much harsh lighting on the subject as possible. Also, if you're shooting indoors enough to require a flash, you'll probably want a tripod. It may be just me, but tripods always seem to be objects targeted for destruction. With that in mind, I always buy the cheapest tripod I can find, as I use them pretty sparingly.
I'm not going to try to suggest a bag. This is very subjective. I carry around a gigantic bag. That's my preference. You may want a smaller, more portable bag. My suggestion is to go to a local camera store and look them over. Buy what feels most comfortable for you to carry around for long periods of time, and will hold the bare minimum of what you expect to want with you.
There are many, many more accessories that aren't covered here. The items presented here are what I view to be the most essential and most used items in a camera package for the weekend photographer. I hope that you can use this guide to help you traverse the sea of equipment and accessories as painlessly as possible. Just remember, a little research goes a long way toward saving money on this equipment. It's worth your time.