Houserule: The Willpower Mechanic
This is a houserule we have been using successfully. The point of this rule was partly to replace action points and the action point economy from 4E that we played at the time, partly to make skill challenges (in general, not the 4E kind) more interesting but also to provide rules that automatically brings narrative and roleplaying into where it is mostly lacking, combat.
The mechanic works as follows:
Each player is granted a number of willpower points equal to CHA mod + 1
or some alternative (WIS mod + CHA mod) or (highest mod WIS/CHA + 1), decide beforehand which method to use.
A willpower point can be spent at any time when a check is failed.
Instead of failing, the willpower gives you another chance at the same task, however with a penalty equal to the effect of the failure (if you rolled 15 agains DC 18 effect is -3).
The next check however is made in the next round, not the same round as the failure. During this time, you are locked in the challenge.
This can continue for any amount of rounds until you either succeed, stop spending willpower or if you fail with more than 10 (at wich point you automatically fail). Note that the DC will change during the task due to the negative effects of the rolls.
Almost any check that is not entirely instantaneous can be extended by a willpower point. Including opposed rolls, saves and attacks. The DM judges from case to case if willpower can be spent.
In addition to the PCs having willpower, some NPCs and monsters may have it aswell. Generally, if the creature has great CHA or WIS, it should get a few points. If it is supposed to be powerful (elites for instace) give it a point for that aswell.
Characters locked in a struggle must use an action (standard action) every round to participate and is partially affected by the failure. DM interprets the situation and describes what a player character can and cannot do while struggling.
The ranger Raln is jumping over a pit. DC to succeed is 18. Raln rolls 16 all bonuses counted, thats 2 from a success.
Now, instead of falling, Raln spends a point of Will. The failure is extended to the next round. The DM informs Raln that he caught a root hanging from the side of the pit, but he is slipping. Meanwhile the goblins on the other side fire their shortbows against their struggling target. Luckily they all miss.
Raln makes another attempt next round, this time against a DC of 20. Raln rolls 22 and manages to clamber up on the edge of the pit, where he can escape into the forest.
The rogue/wizard Nokyal is scouting a temple belonging to a Vecna cult. Dodging the patrolling guards he steps into a chamber, and encounters an acolyte. The acolyte promptly casts Hold Person on Nokyal. Nokyal rolls a save versus a DC of 16 and succeeds with an 19. The accolyte will not surrender that easily however, she spends a point of will. Nokyal is now forced to reroll the save next round. Both are locked into the struggle, the acolyte trying to force the spell and Nokyal trying to repel it, the problem is time; the guards will turn at the end of the corridor and come back. Nokyal saves again versus only 13 but fails with a miserable roll of 8. He counters this with a willpoint of his own, prolonging the struggle. The accolyte smells victory and presses on. At the third roll, Nokyal needs to roll against a DC of 21. Nolyal rolls and gets 24 total, winning again despite the odds.
Released from the struggle Nokyal counters with a Hold Person of his own. The accolyte fails her save and has no more willpoints.
Nokyal shoves the paralysed acolyte under a table and hides behind a statue as the guards pass by. From his hiding place Nokyal sees defiance in the helpless acolytes eyes, he know however that she is spent and he's got a lot of Will left, she is not going to break that spell easily. He picks up a dagger and shakes his head firmly at the acolyte until she stops her attempts at saving.
Other uses of Willpower
In the Hold Person example above. The DM might judge that since the target is a wizard who actually knows the spell Hold Person, that the winner of the challenge is affected by the cast spell (so that there is a chance of reflecting the spell back on the caster). Especially a natural 20 might have this effect.
The willpower resource can be used to replace other similar resources such as action points. In that case, 2 points of will allows one extra action, max one extra action per turn.
Willpower can also be used as a resource whenever it is cinematically apropriate.
"DM: The metal bar is too hot, you must drop it and take 1 HP damage.
Player: I spend a willpower point and hold on despite the pain."
A complete long rest replenishes all lost Willpower. The DM might judge that an incomplete rest restores less willpower and that a rest in a dangerous place similarily restores less.
A few willpower points may also be restored by great successes, good meals, happy surprises and similar events, at the DMs discretion. Similarily they can be subtracted by depressing failures or even a gloomy landscape. This is a great mood-tool for DMs.
Another option is to grant one point of will back everytime a natural 20 is rolled, and to subtract one everytime a 1 is rolled.
For faster recovery, a short rest may restore willpower by 1 (multiple rests in a row does not grant more willpower, unless they constitute a long rest)