Well I've always had problems making that transition with girls, but my awkwardness actually results in some pretty memorable moments for women, depending on the way she's used to see you act you can use your social shortcomings to your advantage. Anyway, here are a few suggestions (consider how long you two have been dating, it's very important) : 1) You could suggest for you to formalize your relationship October 10 2010 because "it's really easy to remember" but you must be very confident of what her answer is going to be and practice is needed so it comes out charming instead of weird. Also throw in a little gift like an origami or something like that. 2) When you're about to introduce her to someone ask her if she would like to be introduced as your girlfriend because you would certainly love the idea.
Also follow the "Friday night rule" (if it's implied you two see each other on weekends she's already your girlfriend). Ask your friends, probably she wants you to ask her the question but you hadn't noticed.