Here is a terrific source, providing exact verbiage, spelling out HOW to ask someone to give you a professional reference.

In a perfect world, you will always conduct yourself in a professional manner, and will leave every job with as much professionalism and integrity that you start it with. 

Your goal should be to always be compiling a list of direct supervisors and co-workers, as well as the occasional client, who are willing to go on record to speak of your abilities. 

The best time to ask for a reference is when you are leaving a job. Asking them to write you a letter that you can use in the future is ideal.

This short article spells out the best way to make that request.  

A well written professional reference letter is often times the tipping point that will be the deciding factor in getting someone an interview. 

This is a terrific source for templates that you can easily customize in order to provide a well written, powerful  reference. 

As a side note, in addition to helping out someone that did a good job for you, another added benefit in writing a reference letter is that it gets your name out there and attracts attention from recruiters and hiring managers. We have placed a number of individuals over the years that initially came to our attention by serving as a reference for someone else. 

This is a post that you should save to use as needed, it makes writing an excellent reference letter a 5 minute endeavor that will result in a top-notch written testimony  that will reflect well not only on the person you are referring, but on yourself as well .