Tell her you have something awsome to show her, then take her to your basement and tie her up.
She'll never leave you.
Works for me every time. My girlfriend's been with me (ie: in my cellar) for almost a year now.
QFT, ive used this method on all my gfs and it works out great.
Sex anytime you want it and if you just put some duck tape on her mouth you won't even have to hear her talk/scream.