Surprise her at work with a box of chocolates and flowers along with a ring. Propose at her desk and then celebrate with her co-workers and a bottle of champagne.
Surprise her by picking her up at the office for a romantic picnic lunch. Bring along a bottle of wine, a ring, and a few romantic words.
If you're a business man, ask her to review a PowerPoint presentation for you and then email it to her at work. Make sure that you are hidden somewhere in the vicinity of her office or cubicle when she views it. Have the PowerPoint display charts and graphs that mark the percentages of your current and future happiness, love, etc. Be creative! At the end of the presentation, have streamed video (or final page) stating where you want this relationship to go. When she screams, appear with flowers and those four special words.
If she has a good-natured boss, contact him/her and ask them to arrange an emergency meeting that day. In advance you can plan to have her family or your family outside of the conference room if you wish. You will be the special guest speaker that day. During the meeting, sweep her off her feet telling her how you feel in front of all her co-workers and hopefully her family.