How to not come across as being stuck-up?

Lately I've heard that some people find me "stuck-up" because I'm not super friendly or open with them. It's really not like that though, I'm just very shy and self-conscious, I never meant to come across that way. I also have been called "frigid" and "cold" which I hate too. It's usually only around guys I get this way, I tend to be fine around girls. I hate being perceived this way but I don't know what to do :S any suggestions?