Beamshots are usually taken in fairly dark environments, so exposure times tend to be longer. So put your cam on a tripod. Use a remote or self timer to eliminate camera wobble.
Make sure you set up your gear in a way that is easy to recreate, so you can easily make comparable shots later.
Use manual settings (as mentioned before), so you can use the exact same settings for every shot. Shutter speed, aperture, ISO value and white balance should be set manually. Lower ISO values are usually preferred, as the pics tend to be less 'noisy' that way. When taking beamshots outside, you might want to use a smaller aperture (bigger number) to increase the field of depth. That way you will get a better impression of sidespill. Inside beamshots are usually taken over a shorter distance and field of depth won't matter as much. Do take note that smaller apertures will need a longer exposure, so the tripod will become more important.
To determine the settings to use, take 3 lights from your collection. The dimmest you have, the brightest you have and something pretty much in the middle. Set up your gear and take pics of the medium bright light with different settings until you get results that look pretty close to the real thing on your computer (don't use the screen on the cam to judge this, a decent sized monitor will work much better). After you have found the desired settings for the medium light, use those settings on a beamshot for the dimmest light to check if the pic isn't too dark. Do the same for the brightest light and check if the pic isn't too light. This might cause a bit of a problem if the difference between lights are very big (like ARC AAA VS. Mag85). In that case you might want to repeat steps to determine multiple sets of settings for lights in various brightness ranges. When you have settings that give the desired results, make sure to write them down, so you always have a reference.
Composition wise, you might want to include an object of known size, so people have an idea of real world scale. I recommend a 2D or 3D maglite for this, as these are usually pretty common to have, both for flashaholics and 'outsiders'. Or you could just put a ruler in the pic.
I'm sure others have some more tips & tricks for you.
