I designed this game about a month ago on the spot with a girl I work with. We constantly refer to it - it's become a little bit of an in-joke. It really works - and I'm going to share it with YOU!
Essentially, the game works by assigning a number for each other on the 'hot-o-meter'. At first, she will be the only one with a number but, as you refer to it more and more you'll get one too. You can start this game right after she reveals something about herself that you find attractive. For instance:
HB: "I'm actually about to start a personal trainer course!"
ME: "Are you serious? Oh my god XX, you just went from here..." [hold hand down somewhere belly button height] "to here on the hot-o-meter" [hold hand somewhere slightly higher] "That makes you a solid eight out of ten!"
HB: "Only an eight?"
ME: "Out of ten! That's pretty good! But I reckon you can go all the way... only if you stop bullying me though!"
Feel free to start them off at whatever number you feel like. The chick I use this on an easy 9 and she's pretty sure of herself. By starting her at an eight, I'm not giving her the perfect score she feels she deserves. Make another reference to the hot-o-meter and that should be enough to make it a little private joke of yours. Move her up whenever she says something you like, or that makes her more attractive. Feel free to move her down too! She'll love it and will try to regain her points. After a couple of references to the game, ask for your own number.
She'll probably say a Number Close to but not exceeding hers. Tell her, "Hey! That's no fair! I'm much better looking than you!". Make sure you smile as you say it! The game is meant to be fun so definitely put your own spin on it. When she starts getting close to the ten mark, tell her that ten out of ten isn't the limit and every now and again someone is just too hot for the hot-o-meter. Introduce the idea of an 11 to keep the game going. Just keep in mind, your rating out of ten shouldn't be much less than hers! Sometimes, move her down to 0.00001 less than your rating. Tell her she's not moving up until she gives you a better rating.
Feel free to mess around with it! It's a great game to play, and it helps sexualize the conversation. Even though it's just a joke, it can't hurt that she'll start thinking of you as a 9 or a 10 out of ten rather than just a friend.