The most important thing about lab work for me is the materials. You need to have all materials ready for students, in some organized way. Sometimes the materials are breakable, or otherwise dangerous. You need to have good classroom management skills so your students don’t break anything.
Couple of points on the outset here. The first would be that the best way to organize anything in the classroom is entirely based on the idea of teacher comfort. If a teacher is comfortable with a particular method or approach, that has to be the guiding force in how the teacher goes about organizing any activity regarding lab work. If the teacher is comfortably, it is an easier and more effective "sell" to the students. Another critical component is the general feel of the students in the classroom. A group that has more overall maturity can be trusted to take more self guided risks and more in way of self discovery within the content. Another group might need more directives during the lab work process. Again, the teacher taking the temperature of the classroom setting becomes a vital ingredient in this formula. The final point to address is whether or not there is a set organizing pattern to lab work instructed by school or local department. Perhaps, there is some type of modus operandi that has to be facilitated in order to proceed with the guidelines of a specific curriculum or approach.
In terms of how to organize students in lab assignments, there are a couple of schools of thought. The first would be that it is best for teachers to compose groups with students that have complementary interests to one another. This will enable, at least in theory, for the most amount of breadth in a given topic to be discovered. Another line of logic is that kids should be allowed freedom to determine with whom they want to work for if they choose the groupings, there will be less focus on antagonisms and more focus on the work at hand. At the very least, it becomes a "teachable moment" if students recognize the failures and shortcomings in working with their friends as opposed to people who will work well in a group setting. Obviously, some hybrid of both is also feasible. I think that prepping the students prior to lab work is critical. They have to know what the intended hypothesis or hypotheses tested will be and how the lab activity connects to their tasks covered in the content. There can be no real effective launch into the lab exploration without this activation of prior knowledge. Ensuring that there are a number of jobs for each member of the group to complete is essential. Who is responsible for testing in the experiment? Who is responsible for preparation of chemicals? Who is responsible for calculating data? Who is in charge of disseminating it? Who is responsible for keeping time? I think that all of these jobs and similar tasks like them can be outlined prior to the commencement of the experiment and in the process, clear instruction and expectations can be evident.