The most effective letters of recommendation are written by professors or work supervisors who know you well enough to describe your academic, personal, or professional achievements and potential with candor, detail, and objectivity. Letters that compare you to your academic peers are often the most useful. Most schools do not consider general, unreservedly praiseworthy letters helpful.
Get feedback. If you’re not sure of your letter-writing skills, or if your letter will carry a great deal of weight for the candidate’s application, ask for feedback from a trusted colleague who may also know the candidate. If you are putting your reputation on the line for this person, you want to put your best foot forward with this letter.