Originally Posted by WonJohnSoup
I think the biggest thing is to have some semblance of a game plan. I would look up and save reference photos for poses onto your cellphone for times when you get stuck. It doesn't even have to be Black Canary pictures, there's nothing wrong with pulling up something similar from another source.
The other may or may not apply to you, but if you're like me, I wish folks would've told me, "Hey, don't even be worried about failing. If a picture looks weird on the LCD, whatever. Have it deleted later and just have fun." So yeah, if you already have a positive attitude, awesome. If you're nervous because it's your first time, you have permission to not have every single photo be 1000% epic. You only need a few =)
Good advice here. Not every photo will be a keeper. Depending on the subject matter and my goals, I usually delete 80% of what I shoot.
Try new things, try silly things, try poses that feel uncomfortable but look good - exaggerated poses and faces are usually the ones that make the best cosplay photos.
One of the things that I had to overcome when I first started shooting portraits, was framing too tight in the camera, and accidentally chopping off fingertips, feet, etc. Shoot loose and crop for composition on the computer later. You can easily crop out excess background, but it's reaaalllyy hard to make the photo bigger and add in some fingertips that you accidentally chopped off.
When shooting environmental portraits on location, pay equal attention to the subject and the background. If the subject looks great, but there's a trashcan or a pooping dog in the background, then attention will be taken away from the subject. You can easily change the background by stepping a couple steps left or right, or by stepping farther away and zooming in.
If you're outside, the best light for portraits will be the last hour before sunset, colloquially called the "golden hour". But that hour goes by really fast and then it's dark, so maybe get there to start scouting and setting up 90 minutes or more before sunset.
 Hmm, some of this is more advice for the photographer, sorry I started typing before really reading your whole post