What Makes a Good Recommendation Letter?

Just write it yourself and let him add or remove what he wants before he signs it.

This is probably the best way to do it. I sometimes ask people to do this when I'm asked to write them a letter, especially if I don't know them well, then I re-word it. The alternative might be to give your boss a list of bullet points of your accomplishments that he might incorporate into his letter.

I start my strongest letters with "I am delighted to recommend John Smith for the position of Widget Quality Control Officer at Company X. I know John extremely well because I have supervised his work for Y years. During that time, he has impressed me as one of the 5 (pick your number) best employees I have had." Then, I go into specific examples. The more specific the letter is, the more useful it is. Include specific examples of your outstanding work, work ethic, independence, creativity in solving problems, leadership abilities, how you went beyond your job description, etc. Quantify your work results where possible. I close a letter for someone exceptional with "I give John Smith my strongest possible recommendation for the position of...".

Obviously, for lesser caliber employees, I'm less enthusiastic, down to opening with "I am writing because John Smith has requested a recommendation for the position of..." and closing with "I recommend you interview John Smith for the position of...". I never agree to write a letter that isn't going to be positive, though I've only declined to write a letter for one person in 22 years. There is always something positive to say.