It seems that more and more these days it is becoming socially acceptable to send an email in place of handwritten letters or cards, and if you're one of those people who just can't bring themselves to pull out a pen and a sheet of paper, don't worry - warming up the love of your life within the cold black-and-white words of computer text can be as easy as using the right words.
If nothing else, you must make your romantic email personal. The recipient has to feel that this email was meant only for them. Anyone can created a canned sonnet, it's up to you to tell them that it's about them. Make mention of something you once did together, mention something about them that you find wonderful - something truly specific. Don't just say 'your hair is really pretty', instead say 'the way your golden hair shimmered in the light reminded me of the way sunbeams cut through clouds after a storm.' Just be sure to let them know what it is about them that pleases you - the idea is to keep it personal.
Take cues from a professional. It might behoove you to dig through your nearest complete works of Shakespeare or Walt Whitman or the Jane Austen novel you never thought you'd crack open. There's a reason these people are still on the shelves hundreds of years later. They have this stuff down. Crack them open for some inspiration or applicable direct quotes. Try not to lean too much on the masters, though. Remember that these are your words, that this email is from you, and the recipient should not feel as if you are not being true to yourself, or subsequently true to your relationship.
Mind the length. If the recipient of the email isn't much of a bibliophile, or might be reading this email in the middle of a very busy work or school day, keep it brief. Just enough to make them happy and brighten their day. However, if your recipient really likes to read and has enough time to read it, write as much as your heart desires.
Above all, be yourself. Let your personality and sense of humor shine through the words. Remind the person you are sending the email to why they love you and what attracted them to you in the first place. The idea of a romantic email is to delight the recipient, and hopefully, keep delighting them for a long time to come.