There are two tips in particular that I have found to be quite effective at increasing the number of photographs that I can take with my DSLR with a single charge.
Let me repeat that... Turn off that LCD! That LCD screen on the back of the camera is really an energy hog. When you don't need it, turn it off.
I leave the LCD disabled whenever possible, only turning it on when I want to review the photos that I have taken. As a result, when I look for a camera body, I look for bodies that do not force me to have the LCD on when adjusting some of the more common controls for the camera or to view the camera's settings. This means learning all the buttons on your camera for the settings that you adjust the most, and disabling the auto-preview option that immediately previews the photograph you've just taken. This isn't to say that I don't use the LCD at all. Quite the opposite. For example, I make use of the histogram for many photographs to check the photograph's exposure. The point is that I only turn on the LCD only when I need to.
Modern DSLRs (at least those with removable lens) are designed so that they can be turned on very quickly, usually taking only a fraction of a second to go from completely off to ready to take a photograph. Furthermore, when they are powered off and then on again, they retain all of their settings as they were when the camera was powered off. When the on/off switch is conveniently located right next to the button, take advantage of that and turn the camera off when you walking between locations or otherwise don't need to be ready to press the button in less than half a second.