Starting with the basics, here's a nice power stance for ya.. Just a little something to warm you up. Everyone loves a classic... BAM!
1) Keep moving
Listen to the direction of the photographer as carefully as you can, but when all else fails- just keep moving. A photographer is likely to capture more intriguing poses if you give them more to work with. It's also easier for them to build on what poses you come up with and concentrate their direction more on finishing the pose. Maybe not every single thing you try will work, but don't be afraid to try it! More movement equals more dynamic images. The perfect flow of posing is almost like dancing in slow-motion, being careful to pause every few seconds and wait for a few camera clicks. As the photographer see's something they like, they'll let you know to hold it or tweak your pose accordingly. Movement is key, movement is key, movement is key!
Good modeling is a workout. Don't go into it thinking it's easy. If your muscles aren't a little sore the next day, you're either Wonder Woman or you're doing didn't fully commit. Poses that may feel awkward and unnatural come across great in a photograph.
Take this shot below on the left, for example. The pose is awesome, her expression is not strained at all and she appears to be almost weightless. But in reality, she's just kind of hovering in front of that car bumper, on an invisible chair, holding herself up mostly with just her tippy toes, legs and left arm. 'Twas not comfortable or natural, I'm sure, but the image is stellar.
Case in point- not every great pose is uncomfortable, but most uncomfortable poses are great.
3) Camera connection is key
Emotional eye contact with the camera is vital. You want to look at the lens like someone is looking back at you. That imaginary someone will vary on the tone you're going for. Maybe you're having a hectic week or still giggling inside from a bad joke your dad told or your team is playing tonight and you're really stoked... But when you can flip the switch, get into your character, and make a connection with the camera- you in turn, are making a connection with the viewer who will eventually take in the finished image. So if you think about it, someone is looking back at you. Not just the photographer, but all the image's potential future viewers.
4) Speaking of hand placement...
While you're in the bathroom diligently practicing your facial expressions in front of the mirror, throw your arms into the mix. Take note of the flattering ways you can frame your face with your hands. But remember, it's best not to actually touch your face so as not to create dimples and smushed cheeks. This also assures that you won't be mussin' up your makeup.
Experiment! Try placing your arms on top of your head, try touching your collarbone/neck/chin/face with one hand, then with both hands. Try hugging your own shoulders, tousling your hair... there's loads of options and the more you practice the more you'll discover. You can find inspiration in your favorite fashion magazines.
Don't ever let a crowd (or others at the shoot) distract you from giving the shoot everything you can. It's human nature to be intimidated when asked to do something that makes you stand out from the crowd. Nerves running amok when modeling in a public place in front of complete strangers is not uncommon. But that reoccurring theme of loosing your inhibitions and getting into character comes to the rescue again. It's safe to assume that anyone who stops to gawk is talking about how talented you are.
Remember; just concentrate on the experience of the hypothetical viewer who waits somewhere in the future to see this artistic image you're in the midst of making at that moment.
Modeling, like any other sport/talent/trade, is something that is developed through experience and practice. You never stop learning, you'll just get better.