For me, the best way to learn to write action scenes was reading a lot of novels by a writer called Dan Abnett, specifically his Warhammer 40,000 novels. That man has military sci-fi action down to a science. I'm at work right now and don't have a copy of his novels handy, but off the top of my head, he writes something like this:

[i]Sidestepping the downward chop of the Ork's great axe, Gregor's breath caught in his throat at the size of the weapon, and the monster wielding it. The Ork was easily eight foot tall, made entirely out of green, rippling muscle, and the blade of his axe was approximately the size of Gregor's torso. If one of the Ork's strikes connected, Gregor was sure he would be entirely disemboweled. However, he was a trained agent of the Throne, a veteran member of the Ordos Xenos, and (most importantly) significantly faster than the Ork. Kicking up dust as he moved, Gregor ducked beneath the whistling serrated edge of the Ork's axe as the beast swung the weapon sideways. Gregor thrust his powersword forward in an attempt to pierce the Ork's thigh and sever vital tendons; when up against a bigger opponent, always attempt to cripple his movement. The blade connected, but before it could pierce the Ork's tough flesh deep enough to cause significant damage, the Ork roared and backpedaled away. For a creature that large, it moved with alarming speed. Gregor wasn't about to give his opponent time to recover and dashed forward. Anticipating the Ork's next move, Gregor flexed his muscles in preparation. He had dodged the Ork's last strike by ducking and, as expected, the Ork attempted to compensate by swinging his axe low, barely a foot off the ground, trying to sideswipe Gregor's legs into nonexistence. With blistering speed, Gregor vaulted over the pitted, rusted axe blade and slammed into the Ork's chest with his feet while he plunged his powersword into the Ork's chest. Grunting in effort, he reversed his momentum, pulled the blade free and leapt off the Ork's torso with a graceful backflip. Blood spurting from his chest, the Ork moaned and collapsed.[/i] 

That's descriptive enough, no?