I'm really starting to wonder if all of the 'Filthy' and 'Dirty' comments are supposed to be jokes or not (And to the ones who are being serious, I hope you realize that you're basing that off of stereotypes that I find somewhat offensive. Offensive because the meaning under these comments is that all men are pigs)
Granted, some men are dirty, yes...but I have some friends who are girls and their rooms are even messier than mine (Which is saying a lot by the way)
Getting back to the point of the thread though.
I agree with what some of the other posters have said.
Think about what makes you feel more masculine and stick with that.
Also, I'd stay away from mementos of your childhood, unless you were lucky enough to get HotWheels during Christmas.
I know for me, that I have lots of little 'treasures' from when I was a kid, that are all very girly, I got so sick of them being in my room that they're now in a box.
I still have them, but they are only for my eyes to see and no one else's.
I would however suggest sticking with darker colors, since darker colors have always been deemed more masculine. Reds, Blues, Greens, Browns, Blacks, those sort of things. I'd stay away from Pastels or really bright neon colors, unless of course, those fit your personal style.
As for a theme. A lot of guys that I know don't have a theme to their room, they seem to just sort of put anything that they like in there.
This of course, is referring to teenaged guys.
As the ones that I know older than that all seem to be married, and share a room with their wife, and of course, their wife decorates (Because women think that they have this inborn sense of interior decor...which I find BS because there are TONS of successful male interior decorators. Wow, apparently I'm feeling very confrontational today)
Anyway, I'll also mention that I'm looking to change up my room too, and I've been looking at different interior design books, and reference material from online. That could help you too.
Good luck!