Find a Feature
Some girls obsess over one or two things they do not love about themselves. It could be anything from the shape of their mouth to the size of their feet. When trying to come up with something sweet to say, find a feature you find captivating that doesn't remind her of the faults she sees in herself. Pay attention to the little side comments she may make about her features. The statement alone may not seem overly romantic, but your words along with a sincere attitude will earn you major points.
Remember a Moment
Try to remember a special moment you had together at a park or during a long car trip. Then, when she least expects it, mention how much you enjoyed the road trip or the conversation about baby names or how close you felt to her during the spontaneous moonlit walk in the park last fall.
She will be happy that you remembered and mentioned a moment she surely keeps in the back of her mind that makes her smile from time to time.
Be Honest and Sincere
People can pick up on subtle changes in tone of voice and body posture. When you want to say something meaningful and romantic, say it with your whole body. An open stance, facing the girl directly, without crossing your arms or legs, is a must. Also, be sure you truly mean what you say. It is better to have a general idea of what you want to say rather than rehearse a written script. There is nothing wrong with stumbling over your words. Spitting out an eloquent monologue may seem romantic, but in reality it tends to come out monotone and flat.