You can't, if you do, your days are numbered and you'll either get tired of lying and fabricating stories to keep her on the side without becoming her official BF or she leaves. I get 3/4 meetings out of a girl before she starts acting like a Khalissi (Dragons an all).
I've had a girl pre-agree with me that she doesn't want anything serious either blah blah...we had one small argument and she throws a line at me "you're just keeping me on the side like your little whore..." girls are not built for this. Unless you meet a married woman who's divorce is too inconvenient and she wants a good physique young guy on the side, chances are you'll be facing a on-going battle.