Walk around the room. A teacher who watches from the corner does not seem very approachable or enthusiastic to students.
Compliment good work. Noticing nice results or improved technique helps keep students motivated in an environment that can be intimidating.
Ask students questions. “How’s it going?” is an innocuous opener. They may answer that things are fine, but they may be encouraged to ask a question.
If you do a demonstration, let students practice. If possible, instead of showing students how to do something, coach them while they try themselves.
If you do not know the answer to a question, find out. Talk to experienced graduate student teachers or to the professor. Giving students wrong information can undermine your authority, and is outright dangerous.
Make sure you know where equipment and supplies are located. Whether or not you are the person responsible for setting up the lab, students expect you to be able to answer questions about the materials they need.
Give students tips to make them more efficient. Students in lab often work under time pressure. Saying something like “You should go ahead to step 4 now because there’s a long line for the instrument you need in step 3,” can help students cope and plan more effectively.
Ask questions to help students understand how to deal with mistakes. A short series of questions (e.g., “What’s the purpose of this procedure?” or “Does this mistake matter for that purpose?”) can model systematic critical thinking.
Help students avoid errors that can cost a lot of time. You may need to repeat an announcement a few times and check on students’ progress more frequently if everyone must get to a certain stage in an experiment or project on a given day.
Know how to handle emergencies. Safety is always a top priority and you should become aware of how to respond both to major accidents and minor injuries. Know where the fire extinguisher, first aid kit, etc. is located.
Relate the lab work to the general coursework. Labs should also serve as a bridge to the lectures and the text, with the aim of reinforcing the abstract concepts with concrete experience.
Enjoy the one-on-one interaction. Take time to get to know your students. Many will view you as a potential mentor.