how to get my girlfriend to cuckold?

Well, my lady first cucked me when I was 21 and I married her soon after! Two years later we're doing great and have some of the wildest kinky sex ever.
Sex and marriage are two different things. I was very jealous and upset at first and we damn near broke up, but we both gave it another chance and got married six months later. We came to an agreement that sex and marriage were different and that she'd involve me from now on, so everything worked out. The jealous feelings fade when you've established a solid foundation of trust. She has sex at least once a week with two friends of mine and I'm cool with it. When we're not having sexual activities we have an absolutely normal marriage. We both love each other. I'm even cool with my friends; they get to fuck my wife and when they're not they're still my friends and we go out for beers and stuff and its no big thing. It's just sex! I consider myself lucky.
Long story short, talk to your lady. Talking can solve a lot of problems and turn cuckolds browniety situations into favorable ones. Take my situation, we talked for hours and hours to work this out! You won't get anything if you stay silent about it.
Oh, and compromise. You won't get any of what you want unless she's getting most of what she wants. I had to learn that the hard way- that's my advice to you. Talk and compromise. Okay?