I've never crossdressed, but I'd probably be good at it. I have a very feminine build and (usually) have long hair.
My ultimate dream is to have a completely androgynous look - I want to basically be someone who people look at and think "Wait, is that a guy or a girl?" and be completely unable to tell until I gave it away by talking or something (one thing I sadly can't make androgynous is my voice). Sadly my girlfriend would never approve... Though she also says I'm very feminine, in attitude as well as appearance.
I probably wouldn't crossdress since I'd be too embarrassed, though. Though I did do this Captain Twilight Lord to You by ~Jutah on deviantART for Halloween this year. (More pictures in my gallery). It's not crossdressing, but it's the closest I've ever come to it (i.e. the makeup).