It's not just about messing with her head; the hot and cold game is downright counterproductive. Either give your time, attention, and care or don't. But, make it clear what you're doing either way.
Don't be on one day and off the next. There are tips and tricks that tell you to do so in the beginning to keep interest, but it's a fine line between initial flirting and begining to engage in a relationship with someone. Once you've met for a date, the on and off again game only harms the future of your potential relationship. It sets the course for doubt, suspicion, and even more games. Vow to pick in or out and stick to your word.