I get that ALL the time. These are a few things I usually say in reply.
- "aw... beautiful AND modest! Its a good character trait, but cut the act... I'm on to you and your ways." From there, just wing it. Say whatever you need to say depending on how she responds.
- "Oh yes you are! But you just dont see it. I totally understand why so I dont blame you. (pause for her curriousity to peak. Wait for her to ask you what you meant). You're jaded by your own beauty! Let me explain. (this is what I say because I have a fancy show car so use whatever story that best suits you.) Alot of people say... wow, your car is so beautiful, its so this and its so that. When I look at my car, sure I think its nice, but I dont get all crazy about it like they do. Then I realized something. Its because I've become jaded... I see that same car everyday, drive that car everyday... its become normal to me. And for you... you wake up in the morning, look in the mirror and see the same face everyday. Brush the same teeth every day. Brush the same hair everyday. What you see now has become normal... but in actuallity, like my car, you are beautifull, one of a kind. Everyone else sees it, but the person that sees it everday. But thats just my theory... but what do I know." Then give them the ****iest grin you can because you know they're thinking "damn, this guy is right!" If she continues to disagree and say she is average, what I would do is say yes... just as my car is average.. and keep on comparing how my car and her are alike. Girls would usually blush, try to fight, but have no way out.
To the others who replied and dont agree with the methods of flattery. I've been using this forever and have always been successful. If it doesnt work for, you... its not the method, but how you are applying it. It works, its just that you're not working it.
One last thing... Its not so much what you say, but more importantly, HOW you say it.