Consider writing the letter in your own handwriting on specialty paper. Companies such as Office Depot, Kinko's and Staples sell specialty paper which adds major impact to your letter. If your handwriting is not very neat and might be hard for a foreign person to read, consider typing it in a nice font or ask some one who has better hand writing to copy your final draft. Include a small photo. Correct spelling is must when writing any letter.
Misspelled words can be a symbols of carelessness.
Take the time to look up a questionable word in the dictionary. Avoid large complicated words when a simple one will do, especially when writing to someone in a different country. They might not speek english as well as you. The opening and closing of the letter are very important. They set the tone for the entire letter.