I've already let everyone know that the light meter is your friend. Now I want to let everyone know how to use it in more advanced ways!
Depending on what mode you're using, the light meter plays a different role.
In Manual, it's just to let you know what it sees.
In Aperture Priority (Av), it's used to choose a shutter speed.
In Shutter Priority (Tv), it's used to choose an aperture.
If ISO is set to Auto, the camera chooses an ISO that puts exposure at +/- 0 EV (ideal), no matter what mode you're in. On Canon DSLRs, you can't set exposure compensation while using Manual mode, even if you have Auto ISO on. Keep that in mind.
Your Brain is More Powerful Than the Camera's Processor.
If you've never experimented with metering, play this little game: Put the camera in manual mode (M), and change the metering to "Spot". Spot metering makes the light meter display the information for the center 5-10% (I can't remember off the top of my head) of the frame.
This means your camera's meter is now a directionally metering light meter ray gun of some sort.
Anyways, once you're in spot metering mode, find a high-contrast scene (Indoors + a window, Lots of Shadows/Highlights, etc.). Start metering the scene (half-press the shutter). Set the exposure by adjusting the shutter/aperture/ISO (don't use Auto ISO) until the camera says it's ideal. Now, move the center AF point (unless you've got AF Point Spot Metering active) to a shadow or highlight. Note how much the meter changes.
By sampling different elements in the scene, you're able to make a decision based on what you're ACTUALLY looking at, as opposed to the camera saying "this *should* do". Examples can be found in many places where the camera's "this *should* do" setting gets it completely wrong. For instance, taking a picture of someone when the sun is behind them. Using spot metering, you can expose their face properly, when Evaluative metering would more than likely expose for the very bright background, leaving an underexposed person.
I don't like Manual mode, and you smell kind of crazy...
Oh, well that's fine. What if you want to take a shot in Av, but it keeps coming out with the subject over/under exposed? Spot metering is your friend! Combine spot metering with Auto Exposure Lock (AE Lock), and you'll be able to tell the camera "hey, expose for this, not what you think you should expose for."
For Av/Tv, I have AF on set to the AF-On button, and the AE Lock function linked to the half-press of the shutter. This means, I can find an object, focus, hold the shutter half-way, recompose, and shoot. The moment I press the shutter half-way is when the camera stops evaluating the scene and changing the exposure settings. Depending on your camera, it may or may not show a second arrow on the light meter that represents the metering in real-time.
For example, if the subject you want to expose for is -1EV different from the rest of the scene, when you focus, AE Lock, and recompose, you'll have an indicator on the light meter at 0 (assuming you have Exposure Compensation off), and another one at +1EV, which is the rest of the scene. As the lighting changes, as long as you have the AE Lock button held, you'll have a marker at 0 and a second one that changes with the lighting.
Share your tips and tricks below!