I agree that you should try to keep it as much a secret as possible. If you haven't said anything to her about it already, I wouldn't. Yeah, you've talked about getting married and started making plans, but if she has no idea whatsoever that you are going to propose, it will make the entire event that much more romantic. Try to play it as cool as possible and not look nervous or anything so she won't be suspicious.
I saw this on a movie and I thought it was kinda cute. The guy basically made the girl think he was getting ready to propose, then he just asked her something silly to throw her off. For example, you could just be sitting at dinner one night and start off with "You know how much I love you, right?" Then you could give her a mini-speech on how much you love her and enjoy spending time with her, etc. Then you could say something like "I just wanted to ask you a question." So by this point she is probably freaking out and thinking your about to propose. Then you could just say something like "Can we get a dog?" Depends on her personality if she will appreciate that later on or not...so use discretion.