The best recommendation letters:

• Are specific to you and to what the writer knows about you. Suggest that the writer talk about a specific course, research collaboration, or interaction(s) between the two of you, and what the writer observed about your work ethic, abilities, and/or character as a result.
• Link to your goals. Tell the writer what your goals are and how this opportunity for which you are applying will help you reach those goals. Ask the writer to discuss your ability to achieve those goals, based on the specific things they know about you.
• Complement the other recommendation letters to present a complete picture of you. Think about the specific things you would like recommendation writers to present about you, and choose writers accordingly. Then, don't be afraid to "prep" your writers by suggesting specific interactions or aspects they could discuss.

Questions about how to choose recommendation writers? Talk to the award's adviser for advice specific to the award you're interested in to learn the process for submission.