Ask each of your references to write a letter of recommendation. There are three reasons for this: (1) if they are not willing to be a reliable reference, they are unlikely to put forth the time necessary to write the letter of recommendation, (2) you will be able to use it as a ready-made reference to supply to potential employers, and (3) a letter of recommendation is a good way for you to prequalify those who will provide you with the best overall reference.
Positive words of praise from others speak volumes.
It is best to allow your references some latitude in developing the letter of recommendation, but for those who are clueless, ask them to follow this format:
This letter should always be typed on the letterhead of the company or institution. The following is a sample letter of recommendation.