I have a photoshoot with David Wenham booked so I will probably come dressed as Eowyn (so it will be Faramir and Eowyn)!
Other than that I think just make sure you wash your hair the previous day so it looks nice, and possibly wear a nerdy T-shirt! I don't think there's any need to try and lose weight just for a photoshoot; you are how you are, if you try to lose weight do it for yourself so you will be healthy, not for a single photoshoot.