I actually had to do this as well, for a very busy prof of mine. Basically I thought about all the contexts in which she knew me, and how I personally felt I had excelled in those contexts, and I wrote about them in strong, glowing terms. I found one of those law school advice books that have examples of "great" LORs and formatted my rec letter accordingly. My prof read over the letter and was impressed with how well it was written, and corrected a few things (apparently she was only a prof for 6 years, not 10 - I had gotten the info from the school's website and it hadn't been corrected).
I think this is actually a great opportunity to say everything you want people to know about you, and say it unblushingly.