A lot of people use
http://www.interfolio.com. It costs $5/letter, but it certainly makes it easier on people writing for you. Interfolio is also convenient for storing letter for later use. It also can guarantee schools that you haven't seen the letter. I recommend it, but you can just have your recommenders send out separate ones if you want. I wanted to make it as easy as possible for them, so I used interfolio. This way I can add schools if necessary at a later time and use the same letters without having to contact the writer again.