club
definitely try the sex club with her (after a few cocktails to get her loosened up), even tell her that u will go just to check it out, maybe have one on one sex just u and her but nothing more. once your there, point out the other women pleasuring themselves with big cocks (any color really), keep your hands all over her gently rubbing, carressing. point out how much bigger the other cocks look compared to u, and ask her how she thinks it would feel with that larger cock fucking her... keep rubbing and carressing. have her look at the facial expressions of the women getting fucked, listen to them moaning, screaming.
anyways, u get my point. it may take more than one visit for her to succumb and really get the fucking she deserves, but once she agrees to watch, hold, suck on another guys larger cock, u are home free. she'll be wanting to fuck other cocks all the time, and the rest will be up to u.
good luck!