Try and avoid the "Rimmer Mistakes" (if you don't know to what I refer, watch Red Dwarf. If you don't know what Red Dwarf is, shame on you!)
- Avoid talking about things you "think" are female
- Avoid distinctive mannerisms of speech.
Also - (though this should be true of ANY gender) - if you're trying a difficult rp, spell well, and use correct grammar (except in dialects). This is crucial to everyone else understanding what your character is trying to be. *grins*