It's probably a reiteration of what has already been said in this thread so far, but the way I approach posing is to simply first play around and explore what sort articulation points are available on a figure. Determine just how much range of motion you have with it, then determine if the weight distribution allows the center of gravity to be stable in a given position. When it doesn't balance, twist or turn something until it's steady.
I start with spreading out the legs, giving them a turn at the thigh swivel if it's there and tilting the ankles. Turn the waist if you can towards the leg that you lead out with if you want the figure to lean forward, or twist the waist slightly away from the leading leg if your goal was to pose the figure as though the character was strafing while shooting. Arms and head position can be freely moved into whatever angle you feel is natural.
At the end of the day, like everyone else said, just use your imagination, but to begin with, you've gotta play with it. Good luck!