Instead of ending your letter with a luke-warm "Sincerely," close with something like "With all my Heart" or "Forever Yours". These sentiments make your closure more romantic and may leave your reader with a warmed heart.
Don't just sign your letter with your given name, but instead use the nickname by which your lover refers to you. If your partner often calls you "cuddle button," for example, close with this too-cute-for-words, unofficial moniker.
Don't include your name at all at the conclusion of your letter, but instead close with some love-themed symbols. Write a line of X's and O's to symbolize hugs and kisses. Or end with a heart in place of your name.
Sealed with a Kiss
In place of a written signature, leave your lip impression, showing your eagerness to pucker up for your partner. Put on some bright lipstick and press your pucker against the paper, leaving a deep and dark impression as a decidedly uncommon signature option.
"Uniquely You" Fingerprint Heart
Create an uncommon and unique fingerprint heart to put in place of your name. Using an ink pad, ink up both of your thumbs. Press one and then the other onto the paper, placing them at an angle and allowing them to overlap at their base so they appear to form a heart. (You'll want to practice your technique on scrap paper a few times to get your fingers' angles correct.)