What would be the proper way to ask for reference letters
Dear Professor Smith:  

My name is Freya ASE.  I was a student in your class on widget design at Enormous State University in 2014.  I am writing to request a letter of recommendation.

After 3 productive years in the workforce at Ballbearings Anonymous, I have decided to apply for the Ph.D. MechE program at CalTech and a few competing programs.  While BA has been great practical experience and good use of my ESU training, now I want to move into cutting edge fluid flow research.

Would you be able to write a positive letter for my applications?  If helpful, I can supply some bullets to refresh your memory on my class project, class performance and our interactions.

Sincerely,

Freya ASE