<p>Try to watch the mannerisms (namely, try not to do anything with the wrist). Be careful with your speech; don't say "Oh, my gawd!!" or "Like, y'know" every two seconds. Whoever told you (or your "friend") to say "dude" more often was completely right. Don't talk about guys OR girls. If a straight guy talks about an attractive girl, give a noncommittal response. Some things you just can't change--like the tone of your voice, or an interest in theater/dance/art (stereotypically "gay" interests). I did all of this for years & nobody had a clue about my sexuality.</p>
<p>But seriously, I suggest that your friend stops being someone he isn't. Sooner or later something will snap and he'll realize what a waste of time it all was. Especially when his friends go, "Oh, you're gay? Big deal."</p>