Letters of love are said to be the most often kept and the most often burnt. It's probably the hardest thing to write even for a professional writer. The whole magic is not the words you write, but the way you write. While reading, the receiver must feel all you feel, your sincerity and your love. It's surely not an easy task, but everything is possible.
As mentioned before, the most important thing in writing a love letter is your feelings; not your writing style or the vocabulary you use. If you are not a talented writer, let it be, write it the way you can, the person will appreciate it as it is, even though it's not a new art treasure.
Of course you can make the most of it. Choose special mail writing paper and don't even think of printing it. It should be hand written. Put it in an envelope. If you have some special experiences together, you may write it on a post card that would make the person remember those moments spent together. It'll make it even more affective. The presentation of the letter is just as important as the content of it.
You should also pay great attention to spelling. Misspelled words are symbols of carelessness. It won't take your time to look up a questionable word in a dictionary. Try to avoid large complicated words. Using simple ones will make your letter more sincere. And will assure you that you used the word in right context.
While writing a love letter, it is extremely important as to what your relationship is like at the moment. If the receiver is just a friend, or you have been dating him or her for a few months or years, or maybe you are already married.
If he or she is your friend, you should avoid too much passion. The letter should express your feelings, but not scare the person. Don't tell that you want to spend the rest of your life with the receiver. You should make sure that even if he or she won't feel the same way, you'll keep your dignity and you will still remain friends.
If you are dating the person for a couple of months, you can tell him or her exactly how you feel. Still, be careful as not to frighten person with too much passion and love, so that even if he or she doesn't feel the same as you do, you could continue dating.
Marriage is a different thing. You can write anything you want, express as much and as strong feeling as you want. If you are married, that means your husband or wife loves you and there's no danger to ruin anything.
The most important thing is to be sincere. Don't push yourself too hard still do the best you can and be very accurate. Don't be afraid to learn that the person doesn't feel the same way. Think of happiness you may have if she or he does.