That's a really tough question. Mostly because PCs will fight their hardest if they see there is a chance they will be captured. This extends far beyond their actions if they think they'll die. I've seen PCs (from a player's viewpoint) actually kamikaze themselves just so they aren't captured alive.
Of course, there is the option that, if the bad guys have access to raise dead, they can just kill off the party and raise them as hostages. Yes, I know that bringing back the dead is voluntary, but in one of the campaigns I play in, the DM has nixed that with Rule 0. This is actually a politicalish typ campaign and city/family based. He's been harvesting our corpses from the graveyard, raising them, and then torturing them to the point they are turned into Meenlocks.
Another option is have Mindflayers around. Mindblast is nasty for stunning the party and tying them up. Course, then the Mindflayers want a snack...
3.0 Enchanters could get their saves for Hold Person/Monster way up. 3.5 Enchanters can, to a lesser extent.
That's about the only thing I can think of right off the top of my head. I'll keep thinking, but as far as I've seen, capturing a PC is one of the hardest things to do.