Asking somebody to be your valentine can often cause feelings of nervousness and worry, as it can cause some people to feel exposed and vulnerable. One solution is to use humor in your approach to lighten the situation, and make both of you feel more relaxed. Fortunately there are a number of funny ways you can accomplish this.
Buy a selection of chocolates for your would-be valentine. Before you present them, announce: "I decided that you're currently far too attractive to be my valentine. Because of this, I demand you consume all these chocolates to fatten up and make me seem better by comparison." If you have the time, produce a picture of your would-be valentine wherein their face has been digitally placed onto the body of an overweight person as "a guide."
Using photographs of you and your valentine, produce a mini-comic book leading up to you asking if she will be your valentine. This can be achieved (using image manipulation software) by digitally cutting out different pictures of you two and placing your images onto a comic-strip background. Create speech bubbles of you two talking to each other.
This can be as long or as short as you like and could cover a range of topics. For example, it could tell the story of you both saving the world from an evil villain, or could give a preview of your Valentine's Day date.
Give your would-be valentine a bouquet of plastic flowers. Announce proudly "If you'll be my valentine, I'll always buy you new flowers after these wilt!"
Hollowed Out Book
Buy a thick book, preferably a cheap one which seems to cover a particularly boring topic. Hollow out the middle of the book, and put several "romantic" items inside, such as jewelry and flowers. Present the book to your would-be valentine as a gift, and enjoy their reaction when they discover the gifts inside the apparently dull present.