Remember that nearly everyone is busy, particularly those who are best qualified to support your petition. Start the process in plenty of time so your advocate doesn't feel rushed. Be judicious in the number of letters you request. Even with the aid of computers, it's a time-consuming activity. To write a persuasive recommendation, a person needs to have all the facts.
Most graduate schools provide a form designed to provide that information.
On the other hand, you may be asked to collect all the letters and then to send them in a single package. After all, you may need help again.