Let me offer a few suggestions.
1. Very specific comparisons are most effective.
a. This student is better than Joe who I sent to Harvard last year but not quite as good as Jane who I sent to MIT year before last.
b. One of the three best undergraduates I've taught in 40 years.
c. Best econ major we have this year.
2. Discussions of evidence of research ability matters.
a. I have recommended this student submit her term paper to the AER.
b. Such a good research assistant that I made him a coauthor.
3. All letters are positive. Specifics are much more important than platitudes.
a. If you can't write a supportive letter, you should probably decline to write a letter at all.
b. If all you can say is that the student got an A in your course and was pleasant and inquisitive during office hours, explain to the student that
everything you know is positive, but the student should think about if there is another faculty member who knows him better.