If you're convinced you see marriage in the future I think it's best to live with each other first for a year. If your not 95% convinced you want to marry her in the near future, dont move in. But if you are, by all means move in together. In a rental. With a contingency plan that will allow you to leave if need be, I suggest having enough money to pay any early termination fee if shyt goes crazy.
The biggest reason to live together is that people can portray themselves in a way that isn't really them over short term periods, and you don't truly know a person until you've lived with them. If after 3 months she starts acting crazy, RUN! If after 6 months you notice things aren't right, slowly start making plans to exit. No one can pretend to be something they aren't for a year, they will expose themselves before then. If everything is great after a year, decide whether you want to spend the rest of your years on earth like this. And if you do, start making serious and short term plans to marry her. Because if you don't she will start dropping strong hints and veiled ultimatums, get fed up and leave, and you lose a good woman because of fear of commitment.
Edit - and birth control, birth control, birth control. Don't move in and knock her up, that's usually what comes next when shacking up. You get comfortable, start bussing inside her, and she WILL BE PREGNANT IN LESS THAN A YEAR