You know, we spend so much time discussing great things to try in our classes we often don't think to mention the things you really should not do. Here are a few examples that occur more than you think--usually because instructors simply don't realize what they're doing:
1. Lack of feedback to facilitate progress. Merely writing a letter grade on a paper provides zero help to the poor student who has no idea why they got such a grade (good or bad). Students need informative, rich feedback on a regular basis so they can actually, shall we say it, learn.
2. Inappropriate expectations of student performance. I've heard cases where instructors will assign a series of assignments over the semester, such as a certain type of paper, and never help students learn how to complete such an assignment--of course many get poor grades at the end because they were never taught how to do better along the way. You cannot presume they should just know how to do what you want--help them along. That's why you're there, by the way.
3. Mismatch of assessment to course activities. All assessment, especially graded assessments (yes, you should have non-graded assessment activities as well) must match the types of activities students have actually worked on in class. Sounds simple, but it happens quite often where students are asked certain types of problems they never encountered, or they are given a multiple choice exam when their coursework included collaborative projects. Assessment should be designed into your course activities and match completely.
4. Ambiguous grading techniques. What exactly does an 82 mean? What does a B+ indicate? Students don't learn from seeing an 82 on their project. They learn by reading and hearing your comments on what they could have done better, what they overlooked, strategies they could have employed, etc. Establish guidelines for what you are looking for and spell these out so they know where the target is. This is what rubrics are for--see the Teaching and Learning Ideas page here at CETL for a discussion on rubrics with an example that works pretty well.
5. Not giving students enough time to think and reflect during class discussion. When you ask a question, and that awkward silence ensues, your instinct is to answer the question and keep going. Don't...let them think a bit. It takes time to process. By the way, don't keep letting the eager first-hands-up individuals to answer all the time. The others quickly learn they don't even have to listen to the questions, much less answer them.
I'll add others as I think of them. Take time to reflect on what the students experience on their side of the fence--what would help you if you were in their shoes?
