If you have had supervision experience in this program, include it either on your vita or in a cover letter or essay. Not all students get this experience, and it helps to make you special. In all cases give them a copy of your current vita, with a listing of practicum and research expereinces that you would like highlighted in the letter. Tell them the kinds of settings you will be applying to so that they don't say you want to work in a pediatric hospital setting when your real ambition in life is to work in a public school setting. If they are able to comment on your thoroughness, on how you have fit in with your peers or with a staff group, and on how you are respected by your peers, co-workers, and faculty, have them address these issues.
Hurried letters tend to be briefer and more general.
Have three or four letters of recommendation sent. Two is generally thought to be too few, and five is too many. You may wish to log all of your telephone contacts with each site, particularly if you are applying to quite a few places. Some places will also ask for your weaknesses, clients that are most difficult to work with, a failure experience from which you've learned. They don't expect you to have everything figured out or to have a perfect record with every client you've worked with.