State how long you have known her and in what way (neighbour? employer?). Mention qualities like intelligence, people skills, punctuality, commitment. Don't forget to say you recommend her most highly.
Work Ethic is a big one. Descibe how you have maybe observed this person doing a job. Also is they are dependable. Talk about them being a team player. Explain that they are a great person to have on your team, or just know in general.