Love letters are read and re-read a number of times. This itself could be a major deterrent when you want to write them. But it is not as difficult as it appears if you keep in mind a few simple things while writing one. Some tips are:
Keep the letters sincere, honest and straight from the heart. If the letters contain honest feelings then they are bound to make an impact.
The opening lines of the letter are important, as that is what sets the tone of your letter.
The tone of the letter depends on the level of intimacy you share with your partner. The intimacy you share with your loved one will help you in deciding how you address him/her. The same rule of thumb would apply while signing off as well.
The content and length of the letter depends on what you want to say in the letter.
The letter should re-iterate the positive things you see in your partner or the good times you have shared.
It is important to avoid any spelling mistakes, because it would show a callous attitude.
It is more effective to send a hand-written love letter on good stationery rather than a printed version on your business letter-head. A hand-written letter always adds a touch of intimacy.
While writing a love letter remember that the presentation is as important as the content, if not more.