One of the least used, yet most powerful, tools you can use to help your future planning is to ask your students for feedback. I am not talking about an end of the course evaluation (though that is also useful). I am referring to a post-assignment conversation about what the students just finished. This is an easy conversation but does require some artfulness on your part in order to get good/useful feedback.
1. Introduce the conversation by telling the students how important it is for you to give them relevant assignments. Demonstrate that you care about how to better help them and hearing their feedback is an essential part of that. If you are discussing a recent test or quiz, tell the students that their feedback will help you design better questions for future tests, which will help you evaluate how much they are learning.
2. Ask open ended questions. Avoid, "Yes or no" questions, such as "Did you like this assignment?" Instead, us something like, "What parts of this assignment did you like and what parts did you not like?"
3. Use active listening. When students are speaking, do not talk over them. Wait until they are done and repeat what you heard and/or ask clarifying questions to make sure their point is clear to you.
4. At the end of the conversation, refer to your notes (oh yea, take notes!). Recap what you heard. Tell them that you will use the information (if you know how, tell them).
5. Actually use the information next time! Remember, you are better off not asking for feedback at all if you are not willing to actually use it.