Fancy new high-def panels offer a myriad of settings when it comes to picture quality, but the two that have the most striking effect are good old contrast and brightness. By setting these correctly, you can get you blacks as black as possible and your whites as bright as possible without losing details in the picture.
Vista Media Centre has a very handy calibration tool - under Settings go to TV and then Configure Your TV or Monitor. After choosing your television type, input, aspect ratio and resolution, you're presented with a Display Calibration tool. The Brightness affects the clarity of dark colours and shadows, while the Contrast affects the level and clarity of whites on the screen.
The Brightness calibration screen shows a looped video clip of a man in a black shirt, allowing you to adjust on the brightness on your television (and/or the graphics card settings). The aim is to get the shadows as dark as possible without losing the finer details of his black shirt.
The Contrast calibration screen shows a man in a white shirt so you can adjust the contrast accordingly, getting his shirt as bright as possible without losing all the details.
Other screens let you adjust the aspect ratio, overscan and colour balance.Some commercial DVD and Blu-ray movies also feature basic calibration tools, such as the THX Optimizer. It's included on my copy of Cars, from Pixar, and you'll find list of movies on the THX website. The THX Optimizer uses test patterns rather than video clips, but the general concept is the same.
When you're happy with your contrast and brightness settings, go back to your favourite movie and see how much extra detail jumps out at you in the dark scenes (The Lord of the Rings trilogy is a good example). Resist the urge to crank the contrast and brightness too high - it might make the picture more vivid but you're sacrificing black shadows and/or a lot of the detail (which is the whole point of having a high-def television).
Image characteristics vary between devices, so you'll probably want slightly different contrast and brightness settings for your media centre, Blu-ray player and PVR. That's why it's important to look for a television that lets you vary these settings for each input.