One of the key reminders for me when planning a boss battle is to have the boss use movement. Bosses who just stand in place are easy targets. The last thing you want is everyone surrounding the boss and wailing away.
Have bosses who heal and use regeneration. Nothing frustrates players more than a boss that heals itself. Maybe in regard to the above reminder about movement, the boss has to get to a certain location to heal up. Hank has some good examples of this in his videos.
I also think a boss needs one or two signature moves that are just terrifying to players. Does you boss cast the Death spell? Does he cause a player to slowly rot away for the duration of the battle? Can the boss mind control a character? Is she completely resistant to magic? Does it show up riding a wyvern? Regardless, have some move that will strike utter terror in your players.
Generally, don’t have your bosses have a ton of HPs. There is nothing worse than turning a dynamic boss battle into a slug fest for round after round. I tend to believe in glass cannons. Make bosses do large damage, but don’t make them crazy hard to take out.
I think it’s also good to have bosses that interact with players in some meaningful way. In one battle, one of the bosses addressed one of my players by name. She was like, “Wait! How did the wraith know my name?!??” She was creeped out the whole fight, especially when it targeted her more than anyone else and was immune to her magic. In another battle, I had the boss show off an epic piece of gear and then challenge a player to essentially a one on one. The character was more than willing to take the bait and try to get that awesome piece of loot from the boss’s corpse. If you make it personal, players will want to kill them.
Anyway, those are a few of the tactics I try to employ. Have fun and be sure to circle back and let us know how it goes.