I was thinking a letter, too. That way the person has to 'listen' (most people wouldn't be able to ignore a letter) and it lets you gather your thoughts and get your message across as best you can.
Also, "I" messages. I feel... otherwise it becomes blame (even if you're not wrong) and the person feels/becomes defensive. You want them to understand where you are coming from.
Friends aren't people you particularly like for any special reason. You just like people because they're your friends.
Rosamunde Pilcher, The Empty House