As a high school teacher I was asked to write a lot of college recommendations (not so much as a high school librarian) and here is what I learned.

1. If you cannot write a good one say no. Say it nicely but say no. And do not damn with faint praise. In that case you should have said no.
2. Double check if the letter is a free form letter or a form you need fill out. You would be surprised how often people forget to mention these things.
3. Ask the person for a list of things they have done or accomplished. You want your recommendation letter to be as specific to the individual as possible. If you fill your letter with general platitudes you might as well not have written anything.
4. Be honest. If you think there are some weaknesses in the persons candidacy talk to them before you write the letter. Let them know what you are going to say and see if they want you to proceed. Letting the reader know of weakness is not the same in my book as writing a bad recommendation. A reader once told me that they appreciate and read those letters more carefully because they know that the writer is taking his job seriously.
5. Give your contact information if the reader wants to follow up. It has never happened to me but you never know.

When I have done this, I use specific examples from what I have observed of the person to illustrate why I believe they would be a successful student (attention to detail, curiosity, enjoys challenge, etc.) and why they will be a successful librarian after their degree. Brief, but specific.