Don't drop hints that you're keeping a secret. In other words, the "I know something you don't know" game won't fly, especially if you've been dating for a long time and she thinks a proposal could be in the works. If you flaunt the fact that you're hiding something, you might as well just keep the ring out on the coffee table. If you're overwhelmed with the urge to tell your secret...
...don't tell a person, tell your dog. Or take a drive by yourself and say it out loud, scream it into an empty field, tell your stuffed animal, but whatever you do, don't tell anyone unless you need their help to pull off the proposal (like they're responsible for getting your girlfriend to the proposal place, or filming the moment from afar). The more people who know, the more likely it is that the secret will spread.
No matter how you plan to propose to her, do it in a way that doesn't alter her normal routine. As in, don't ask her to call in sick on the day (or the day after) you plan to propose, don't tell her that you made reservations for the best restaurant in town, and don't tell her that she should save her vacation days for later. She'll know something's up.
When it comes to buying the ring, pay with a card that she knows nothing about. That way, there won't be any chance she'll stumble upon a bank statement with a record of your purchase. And when you do go to buy, don't give the jeweler your home phone number (especially if you live with your girlfriend). Use your cell as your contact number instead and save the jeweler's number in your phone as something generic, like "Mark Jones" or "Jim Brown."
Find a good hiding place for the ring. Don't leave it in the glove box of your car, on top of a bookcase (unless she's really short), or in your medicine cabinet. Basically, try to find a place she's rarely near. Some ideas: in a piece of luggage you won't use before the proposal, in your toolbox, or in a pocket of some off-season clothes (swim trunks if it's wintertime, a winter coat if it's summertime).
The day of, week of, or even month of, you're bound to be nervous. But don't let your pre-proposal jitters give it away. Rather than pacing the living room, find an outlet for all that pent up energy. Go for a jog around the block, get a massage, and practice deep breathing to keep nerves at bay.