do you mean the total principles?
the most important fact is to be truthful. try not to praise that person too much , or more than he deserves, and not criticise him mercilessly even if you dislike him.your aim should be placing fair and moderate judgement on the person you are recommending. at the end of the letter you either recommend him positive for the position he is applying for, or you state that, in your opinion, he is unable to fullfill the duties of the position for which he has applied.
note that the most doifficult letter is a letter of recommendation..it requires a great deal of thought and consideration, and requires that you know the person you are recommending well.it is extremely difficult to be completely fair and impartial towards a person you may like or dislike very much.you should be moderate as well. the reader of the recommendation should be able to believe you, otherwise your letter is worthless.
hope it helps