Rent a billboard and drive by it one afternoon while you're together. You can contact a large company such as EMC Outdoor that offers billboards throughout the US and Canada. You could also check the yellow pages for local billboard advertising.
Put your proposal up with the ads before a movie and take your sweetie early to see the show. You can contact your local theater for advertising information or contact a large theater such as Regal Cinemas for advertising information.
Sky write it. Contact a local skywriting company and plan to have them display your proposal on a clear afternoon while you're out. One idea... spend a Saturday afternoon at the beach (or a lake?) and arrange for the skywriting while you're there.
Have a plane fly a banner. Like the skywriting, you need to make sure you do this on a clear day. Plan a romantic outside date and they'll be pleasantly surprised when you ask what that banner says as the plane goes by. Your best bet in setting this up is to contact local companies to see if they or someone else provides this service. Arnold Aerial offers this service nationwide or check your local phone book for a local company.
Put an ad in the local paper. This is incredibly easy to do and very inexpensive. You can place a classified ad, such as 'Seeking single female/male named such-and-such to spend her/his life with me'. Or, you could up the ante and place a larger graphical ad with a picture of the two of you and your proposal. Contact your local paper for prices and details.
Have it displayed on the scoreboard at a ball game. This is a really cool way to propose, just make sure your girlfriend sees it! We've all seen the commercial where the guy proposes on the scoreboard and the girl misses it because she's eating a candy bar. If you want a memory, have a friend nearby to video the proposal and her response!
Paint your proposal on the roof then take her flying over it. You don't necessarily have to use paint, though it may be easiest to see from above. You could try using bricks to spell it out, chalk, or something else. Contact a local company that takes tourists in the air and ask them to fly you over it.
Do you live in a dorm or apartment building? Get neighbors to turn on/off lights to spell the words "Marry Me" and then take your beloved across the street for a surprise.
Take a charted hot air balloon ride for two with champagne. While up, have a chartered banner tow plane to do laps around the balloon with your marriage proposal message behind airplane. While sipping champagne, she'll be surprised when the plane comes by. Another idea would be a skywriting plane while you're up.
This idea involves a rather grand setup on a beach, but is well worth it. Choose a local beach site that is quiet and not that windy. Prepare a lot of candles (bulk candles in steel cases such as votives). Place the candles on the sand to create the words "Will you marry me" in very big letters. Light them up in the evening and have someone watch over them to make sure they don't burn out. Take your girl to the site "for a walk". She will be surprised to see the words glowing in the dark. Ask her to marry you in front of the grand scene and hopefully she will say "yes"!
Write 'Will you marry me?' in luminaries on a hilly road that you can drive up and see.
Take her out for a nice dinner and to a movie. In the middle of the movie, arrange for the staff to stop the movie and announce that they are having technical difficulties and it would be appreciated if everyone remained seated. Then, have them role a slide show of photos of you two together playing "your song." At the end of the slide show have a video clip of you talking and saying how much she means to you, etc. Next, ask her to marry you. When she turns around to look at you present her with the ring.
When you're at a place where horses are involved, such as a track, arrange to have 4 horses ride by with the words will you marry me on their saddle cloths. Each horse should have a different word printed on their saddle cloth. Alternatively, you can have 5 horses and have the 5th horse with the persons name printed on it.
Carve the words 'marry me' in a corn field, then fly her over it. Be sure you have permission first. :)
Make signs several signs with one word of the big question on each sign. Then, starting with one with your sweeties name on it, place them on his/her way home from work. When they get home, be waiting with your romantic plans for the evening.
Have a large banner made by BannerGram and surprise her with it over your garage when she arrives home.
If you'll be picking your sweetie up at the airport soon, give this one a try. Plan ahead to have friends and family at the airport with you waiting for her arrival. Create a large banner with the words "Will you marry me?" on it and have everyone holding it up with you at the 'me' end. Or, create several small banners with one letter on each spelling out "Will you marry me?" (again, with you holding the me?). When she arrives and sees her surprise, get down on one knee and propose.