When writing a love letter, be honest first and foremost.
Someone who really loves you will probably be able to tell if you are faking it. If you're the flowery type, let your letter overflow with purple prose and embellishments. If you're more plainspoken, be plainspoken. Refer to your own little secrets and personal in-jokes that only the two of you know. Make sure she knows it's only for her, from only you. Avoid cliches and generalities, as they are clumsy and unromantic.
Stationery and Ink
Never type a love letter! Write it by hand even if your script is twisted and nearly illegible.
Do your best to make it legible, but a love letter is not a job application. When it comes to romance, the intent and meaning behind the letter are more important than its readability. Use nice stationery if you have it, but that's optional. Your lover will probably prefer a letter written on hand-decorated typing paper to one on a cheesy and impersonal Hallmark card. And always write in ink.
Subtle Delivery Ideas
If you're the more subtly romantic type, consider sealing your letter in a nicely decorated envelope with a few rose petals or squirts of cologne inside.
You may wish to split your letter into several smaller letters and mail them one at a time, so they arrive on different days. This spreads out the joy of getting a love letter over a period of time, which can be quite romantic.
Wild Delivery Ideas
If you prefer to dress your love in brass band music and bright lacy hearts, consider something a little more hardcore romantic.
Save a wine or beer bottle, place your rolled-up letter inside with a few construction paper hearts or some glitter, and mail it to your lover in a box. Sand and shells inside the box are optional. Mail a rose with fortune-cookie-sized love letters rolled up in between the petals.
Stay-at-Home Love Letters
Even those not in long-distance relationships can write each other love letters.
Place short love letters in interesting places around the house, such as rolled around your lover's toothbrush, in the cap of his bottle of shaving cream or taped to her favorite coffee mug. Write sweet nothings on the bathroom mirror, on the labels of beer bottles or even in washable ink on his hand while he sleeps (if he is a heavy sleeper).