Turn on AE Lock (Hold)
This is a personal preference of mine, but you might like to set Custom Function f4, Assign AE-L/AF-L button, to AE lock (hold). This way, when you want to lock the exposure on the D90 (which is especially useful when you don't want the exposure to jump all over the place while shooting movies with the D90), all you have to do is press the button once, and you can let go of it - the exposure remains locked. To have the camera re-meter an exposure, simply press the button again.