Buying the right DSLR from the onset is important. If you pick the wrong one, you'll most likely end up replacing it down the road and in many cases, it may actually discourage you from taking pictures in the first place.
The first rule of thumb is the most basic one: buy what feels comfortable in your hands. If the camera does not feel like an extension of your arm, try another. The fact that your best friend/father/sister shoots Canon/Nikon/Pentax is irrelevant to you. Keep trying cameras until ergonomically, you feel "at one" with the camera. Your fingers should be comfortable and fall naturally on the buttons, your wrist shouldn't be sore, and the software and hardware should be intuitive. Remember, a DSLR is not a "pocketable" device and if you plan on using it, you will carry it for long stretches at a time. So comfort is king. Else, you won't use it: you will miss photo opportunities because you'll in all likelihood make up excuses to not carry it and the camera will eventually gather dust.
The second rule of thumb is closely tied to the first one. Not only does the camera have to be comfortable, but it must suit your needs: what do you want to take pictures of? How will you consume the pictures afterwards (i.e. facebook, flickr, printed etc)? I tend to shoot in somewhat challenging environments (i.e. dark, humid, crowded etc) and like to print my pictures on 3 by 2 feet canvases. I therefore own a camera that can withstand the abuse, that can perform under some difficult circumstances, and that allows me to enlarge images for display. For most however, my camera would be complete overkill: it's big, it's heavy and it's incredibly flexible to the point that it sometimes confuses me! Before walking into a store, figure out what it is you want to do with the camera that way you can ask the right questions to the salesperson from the get go. The salesperson will be able to narrow down the options (note: not all camera stores carry all DSLR models: look around online for options and reviews). Before moving on to the next point, note that there probably isn't a single camera that can meet all of your needs. For all of its advanced features, my camera is often simply too large to carry or is a theft magnet. On my most recent trip to Brazil, I chose to bring a much smaller and discreet camera. I knew I would not shoot in my usual challenging environments so I was willing to compromise.
The third rule of thumb has to do with the single most important piece of the DSLR puzzle: the lens. The kit lens that comes with your camera is a jack of all trades, master of none, lens. Want to take a picture of your child's recital in a dark gymnasium? You'll be disappointed. Want to take pictures of a fast moving person in low light? You'll be disappointed. Want to take candid shots during the holidays in a well light room? You'll do just fine. The sooner you know what you want your camera for (see second rule of thumb), the sooner you can make a better decision on which camera to purchase and the type of lens you should buy with it. Do not compromise on the lenses. In the end, you'll end up buying the lens you should have purchased in the first place, thus wasting more money. That kit lens is massed produced, it is often replaced, and is therefore worth next to nothing on the used market. Pro or advanced amateur lenses? The complete opposite.
I can personally attest to the three rules I listed above since I have essentially lived them. I received a Pentax as a Christmas gift, but quickly switched to Nikon because the camera felt better in my hands. I have since acquired several different models, each "meeting the need of the day". I also fell into the lens trap like many do, but have since "figured it out". My final advice for you is that once you find the perfect camera, go out and experiment. Be creative and have fun.