Fortunately, I have no problems with my players when it comes to kidnapping - they don't usually fight to the death unless the character concept warrants it, and the players don't go all-out to prevent a kidnapping.
Good methods to kidnap PCs include:
- Hold person
- Poison, especially sleep poison like the drow use (Although some PCs will resist the poison)
- Charm/Dominate Spells
- Guile (It is impressive how far a PC will lower his or her guard when it comes to the bedroom. Some kinkier PCs looked forward to getting tied up. Another fine trick was the halfling disguised as a little girl, crying due to a scratched hand, asking the big bad fighter to "kiss it and make it better" - and when he bent down, wham, blinding powder in his eyes, and out came the poisoned dagger. Just telling them to enter a room and wait there, and then lock the door works as well as long as the door and walls are sturdy enough to prevent escape. And there was the day our group met with a prospective employer in the baths, naked and without weapons, and it turned out to be an ambush.)
- Direct Damage spells with the subdual substitution feat applied to (Fireball them into unconsciousness).
- Big bad Monsters that grapple and Pin, with a couple henchmen to tie the grappled PC up (after stripping them of everything).
- Strange magical puzzles (I once had a door that required the removal of all riches to pass it - the PCs stripped down to undergarments to enter, leaving a hired ranger to guad their stuff, which turned out to be a traitor. Could have been a magical prison as well.)
In order to prevent untimely escapes or suicide attempts, kindly remove everything useful from the captured PCs. In my campaign, this often includes any clothes - you never know what might be hidden in the seams of the undergarments of a rogue.
And if you really want to play hardball, make the bad guys use the gear of the PCs, don't have it stored next to their cells. My PCs were once very ticked off when they lost a fight and discovered that their expensive healing potions had been used by the enemies to recover, and that their gear was already distributed among the survivors ort sold off.