Always have an open forum after every session. Ask them what they learned so far and let them ask you questions about the things they're still confused about. It is very important that they fully understand everything on the MSDS.
Verbal training is of no use unless employees don't know the practical application of tools. Hence there should be more focus on "on-the-job" training so the workers can understand the real-time challenges and how to deal with it.
I make the staff look up a chemical that they work with, tell me what's in it, the hazards, and how to clean up a spill. It forces them to at least learn the process.