Study the classics. Romance movies are some of the most compelling stories. Movies such as Casablanca, Gone with the Wind, Romeo and Juliet, and Breakfast at Tiffany's are just a few great love stories that you should watch. Pay close attention to how the story develops and to the most passionate moments in the film. Take notes and remember what you've learned for your own screenplay.
Practice creating love scenes. Take two characters and write -- in a paragraph or two -- a breakdown of a romantic comedy screenplay. Then, take the same two characters and envision a romantic tragedy. Think about the differences in mood and the setting that the characters exist in, and how the love scenes will look dramatically different in each scenario. Write a broad vision of the scenes, as well as your ideas for character nuance, setting and even music. Listening to romantic music will help inspire you to write a love scene -- and if it's a song you envision to be part of the film, then include it as a notation in the actual script.
Create a romantic mood and setting for your screenplay based on what would work best for the type of story you're telling. Maybe your characters are two nobles exiled to a foreign country with war and terror raging around them, or they could be two Midwestern teenagers in high school realizing the joy and pain of love for the first time. The setting will enhance and inspire the connection between your lovers. In the examples above the nobles would be tense and fearful because they are in a dangerous setting -- while the teenagers will be fearful due to lack of experience and social expectations.
Focus on the details of your two lovers. Ask yourself how are they both feeling and consider the motivations of the characters. Think about what is at stake in their relationship if they do or don't get together. Having a good understanding of the consequences involved will help lead you to a well-written love scene.
Show, don't tell. You'll hear this in any writing class, but it's especially pertinent in screenwriting. Film is a visual media and the expression of emotions does not always happen verbally. Let your readers see, feel and imagine how your characters must be feeling. Create an exaggerated awareness in your love scenes -- including small details such as flushed cheeks or sweat on a character's brow will add sensuality and realism to your screenplay. The emotional impact of the two characters can be expressed in every gesture. An example of this is a moment of accidental contact between two characters when one touches the other. They can exchange a long look brimming with sexual tension. Use little moments of expression like this to build tension for the climactic release -- which is the highest point of tension in a screenplay.
Act it out. Once you have written your love scene, have some friends read and "act" out the scene. Read all the narrative too. Doing this can tell you a lot about what does and doesn't work for the scene.