Know the fights, know the fights, know the fights. Make sure you explain them well.
Caveat: do not go into every single minute detail, spending 15 minutes explaining a fight (especially one with three or more phases). Many players will tune you out, and are probably "active learners" (actually doing the fight makes stuff sink in better). Detail the first phase, go over the second, gloss over anything more than that. If you have one or two new participants explain the fight in terms of what they need to know based on their role (i.e. DPS and heals do not need to know the intimate details of tank swapping)