Inform yourself about the person you are recommending. Spend some time talking to them about their goals and the position they are applying for. Gather together their current resume, any memos or notes you may have on them, and any other information that will help you as you write the letter. The best recommendations are thorough and specific, and having all the information at hand will make the process much easier.
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.