Zet: Dittoing Shi. Compression shirts are a godsend. Totally worth the investment if you plan to cosplay guys regularly.
Also, just as a sidenote, if someone actually gropes you at a con, drag them over to a Yojimbo and explain that they've been sexually harassing you. Seriously. That sort of behavior is totally inappropriate, and shouldn't be treated as a norm. Even if they were "just playing around" and look seriously contrite, they at least deserve to have it explained to them why sexual harassment is not okay, even at con.