So I've DM'd three separate games, of which one that I'm largely happy with. However, one consistent problem I run into is designing fun, exciting, and interesting boss fights. I've run into two major problems.
The boss fights are over too quickly. Generally this boils down to a problem of the players winning initiative overwhelmingly, and bum rushing the boss. This is an older problem, and one I've been able to largely solve, but I've run into it once or twice since my first mistake with it. I've added minions, generally my boss enemies have Improved Initiative. Still, it happens when I underestimate my party's damage-per-round.
This problem is more commonly occurring. Fights turn into slogs. The boss doesn't get hit (usually from either a high AC, a lot of HP, or some other defensive mechanic like miss chance or some such). This is even worse than the Blitz, because instead of saying "that was easy," it boils down to "that was boring and a waste of my time." This can also happen from status effects like Fear, Hold Person, and other similar Save-or-Suck effects. But I don't know how to make a boss intimidating (and ergo, exciting), without using the strong Save-or-Suck effects.
For example, just recently I had my gaming group fight an evil cleric, in which I planned using minions, but thanks to unforeseen circumstance, the minions were quickly eliminated by the party's cleric. I planned on using them to harass the players while the cleric boosted the minions, so he had a lot of status effects. Unfortunately, when those minions were eliminated too quickly with Channel Energy, and the strategy was shot. He had a Deeper Darkness spell as a last resort, but it ended up bringing the fight to even more of a slog.
So my question here is twofold:
What specifically am I doing wrong, and what can I do to fix it?