Hey there! Photographer here! Hopefully I can help out with this, since this is a huge part of my responsibility on a shoot.
So first of all, let me just clarify that it's one hundred percent normal to feel this awkwardness about posing for photos, especially if you're not used to it. So no matter what, don't blame yourself for it. It happens to almost everyone.
Anyway, let's move on to how to try and help the situation. First off, I'll go ahead and reiterate what everyone else in here has said: shoot with people who make you feel comfortable. Friends are a great place to start, since you're already comfortable around them. However, a photographer can also do this. All you have to do is talk to them and get to know them in the days and weeks before the shoot. If you find a photographer whose work you really like, meet up with them (be safe please - do it during the day at a public place and bring a friend). Once you meet them and get to know them a bit, hopefully you'll be able to see if you feel comfortable around them and if you can be yourself around them. If they can make you feel relaxed when they're just meeting and talking with you, it's a really good sign that they'll be able to do it when you're shooting.
Apart from that, a lot of the previous suggestions are excellent. Practice posing in front of a mirror (ideally a full-body length mirror so you can see a head to toe view), and try to think of things that your character typically does. If you're cosplaying Master Chief and you're not doing a weird spin on it or a humorous shot or two, chances are you're not gonna be rollicking in a field of flowers. Allow yourself to role play as the character rather than yourself wearing a costume, and try to keep in mind how they would act and what they would do. Practicing your makeup in advance is also a fantastic tip that's sure to save you a ton of time and trouble come con / shoot time.
Apart from that, just remember that it's primarily the photographer's job to be the eyes. They should be able to relax you and direct you into good poses, while keeping an eye out for little details that can improve the shot and make the pose look more natural. Also, one thing to remember is that there are definitely poses that feel weird when you're doing them, but look fantastic on camera, so as long as you're not uncomfortable, trust your photographer. If you can relax and trust the photographer, you can then focus on just being your character and playing around with the costume, location, props, and character. You'll have a of fun that way, and don't be afraid to have all that fun! If something makes you want to burst out laughing, let it out! You can put the pose back together once you're done and get the shot again, and you'll have some wonderful bonus candids if your photographer was paying attention
Hope this helps and that you have a fantastic time shooting!