Here are some tips:
-Put a picture of your significant other in front of you to help inspire you while writing.
-Put on some light romantic music, or read some romantic literature to get you in the mood and give you some ideas on what to write about.
-Write a rough draft first and then put it onto a final copy. A grammatically incorrect or misspelled letter can interfere with the message the letter is trying to convey. Also the presentation of the letter is just as important as the content.
-Write your final draft on nice stationary.
-Write from your heart, be sincere about what you say.
-You should open up a letter with "Dear" if you are early on in the relationship, but if you are at a deeper stage, "My Beloved", or "To My One and Only" may be more appropriate.
-Don't close your letter with "Sincerely", use, "With warmest regards" (if you are at an early stage of your relationship), "Love", "Yours always", "Your beloved" (if you are at a later stage of your relationship) or something else along those lines.
-Wait at least a day before sending out the letter, just in case you wrote something that you may regret later.
-Put a small dab of cologne or perfume on your letter before you seal it up and sending it out.
-Usually people only put a date on for the heading of the love letter.