It depends on the kind of woman you are proposing to. By this time, I suppose you know what she is crazy about- plan something around it.
1, Recreate your first date. This will bring back a flood of fond memories and will put her in the right mood for your proposal.
2. Plan a surprise picnic. Consider taking your beloved to the woods or to a Southern California beach for a romantic picnic. At the picnic site, you might consider spelling out the words "Marry me" with stones or seashells.
3. Bring out the bubbly. During dinner, serve your girlfriend a glass of champagne with the engagement ring in the bottom of the glass. You’re then in a perfect position to toast your future together. It’s hard for a girl to pass up a ring when it comes in such elegant packaging. (One fun and elegant location to have the bubbly served is at an upscale Las Vegas hotel. The lights of the strip at night coupled with the glamour, fun and upscale service that can be found in Vegas, might just make the perfect backdrop.)
4. Pop the question in a cookie. Consider dining together at your favorite Chinese restaurant. Have the waiter serve her a fortune cookie with "Will you marry me?" hidden inside.
5. The Christmas tree proposal. At Christmas, invite her over to your place to view your Christmas tree. Make sure that the tree is lighted, but has only a single decoration—an engagement ring.
6. Spell out your proposal with style. Get her a gift which spells out your intentions (M-A-R-R-Y-M-E) in a charm or a bracelet. Around dinner!
More here:
http://searchwarp.com/swa63885.htm
http://base.google.com/base/a/1214205/1310722969417622724
http://www.buzzle.com/editorials/5-27-2006-97494.asp
http://www.acbouquet.com/marry_me.htm

I proposed to my wife in front of 800 people. She was playing Belle in Beauty and the Beast, a show that the local community theater was putting on. On the last night, I got permission from the director to propose on-stage in front of the audience. She had no idea it was coming, and she now has memories of being proposed to while wearing the yellow ball gown that Belle wore in the movie.

I think the most romantic proposals are the ones that really reflect your personality or your fiance's, or really mean something personal to you as a couple. For example, I would find it really cheesy if someone proposed to me at a sports game, but if you're both huge fans and at a playoff game or something, it might be the greatest thing in the world for you. One of my favorite proposal stories was a couple who both really loved a particular band and got the lead singer to announce the proposal at a concert.