does powerful have to equal "power" or can it also be read as "striking" or "powerfully emotive" or something like that or are you limited to somehow showing power.
One way to make someone look looming and powerful is both through posing and angle. If you shoot from a lower angle, the subject may seem to loom over you and certain poses imply power.
Also, not sure how good you are at lighting, but lighting can play a very big role in showing power.
Here are some examples I found using flickr that may help. Use flickr for inspiration, flickr is your friend!! All the below took 2 min.
on Flickr - Photo Sharing!
on Flickr - Photo Sharing!
Confidence on Flickr - Photo Sharing!
A guy with whom you should not mess on Flickr - Photo Sharing!
Macho on Flickr - Photo Sharing!