As daunting as it may seem, getting your photo taken at a con is not a really big thing.
An experienced photographer will usually help you with body position, how to stand and for the most part basic modeling, but there are a few other basics to remember:
1. Attitude shows- if you ARE
nervous, distracted, anxious, rushed or otherwise "not in the moment," it will show.
If you don't have time for a few photos then decline. If you don't feel that you are prepared for a photo but still want to have one taken, ask the photographer for a few moments to get composed. Don't worry, they'll wait, or ask to meet later.
2. Don't be afraid to offer suggestions- You'll likely know more about the character than the photographer, so you will know the look of that character and it's signature poses.
This type of photography is as much about cooperation as it is about cosplay.
3. Know your limits- Before you even go to the con, know what you are willing to do and what you can do.
Don't allow yourself to be pressured into any photo, photoshoot or actions you are uncomfortable doing. This is one of the few things that there should be no compromises.
And if you feel that your cosplay can't handle action shots, then don't do it.
I've seem more than one instance where a costume literally came apart while trying to re-create a combat move or even just running.
Dom Bower recently did a very good video of a first-time model. While it's not strictly a tutorial, it shows how a professional handles himself while working with a novice.
I'll probably add to this when I'm not as tired.