1. Put in what you want the students to do, as specifically as possible. It is no fun to be trying to guess what the teacher wants.
2. Say things in a positive manner. “A successful student is on time.” beats “Don’t be late” in anyone’s book.
3. Include contact information with times that you are most likely available.
I give my students my home phone number and ask them to call only between 9 am and 9 pm. In all the years of teaching I have only had one student call at any other time. (I did have a parent of one of my high school students call outside my time and she did it on purpose, whereas my college student just picked up the phone and called when he got to a part he didn’t understand. Of course, he was studying at 2 am in the morning. I offered to call him back when I got up for my 7:30 am Saturday class.)
4. Start with the end in mind. What do you most want your students to know/do? Begin there.
5. Break long assignments into multiple shorter ones. Most people will be happier and the outcomes will be better. (For a long research paper, I start with choosing a topic, then move into finding sources, then have them take notes, etc.)