I briefly used once a Luck system, superficially similar to Hero points. It promised, but needed some practice to use it well, and work to round and finish it. It goes as follows:
There are those situations that while somewhat common are not defined in the rules, and their arbitrary inclusion could strike the players as, well, arbitrary. Horses lose horseshoes, the inkeeper's mood may be hellish becaue he has a toothache, weapons break, it rains when and where it shouldn't or your tailor found you, waving with a big bill. On the other hand, the sentinel may think it's time to pee when you approach, there could be a forgotten and somewhat rusty dagger hidden behing a loose brick in the cell's wall, you can find a forgotten friend (who has a warm fireplace and some information) or, you know, have good luck. The kind of good luck you it makes you think your DM is a little too generous.
Using the Luck system you award a Luck chip to the player most directly affected by it when something arbitrary and bad happens. He keeps the chip and can spend it when he thinks he needs some extra help; he declares in general terms what he'd like to happen, or the benefit he'd like, and the DM decides the final outcome. In case several characters are equally affected by the bad luck, randomly decide who gets the luck chip.
Luck chips are only awarder by unscripted outcomes not directly derived from the adventure's plot. For example, if the bad guy Scrys the party and ambushes them, it comes from the bad guy's capabilities and no Luck is "awarded". The same if the bad guy is rich enough to hire a wizard to scry the party, assuming there are available wizards for hire. But if the bag guy is poor and you decide the party was unlcky and a bag guy's henchman saw them entering their hideout, or no casters for hire are readily available and it's a matter of pure bad luck the bag guy got him, then the luck chip is awarded.
Also, avoid game mechanics when deciding the result of a Luck event, good or bad. A bad luck event should introduce new obstacles for the party to overcome, instead of making existing ones more difficult by introducing a mathematical modifier. As an example, suppose the heroes are fighting on a ship, after rescuing the princess. You can tell the hero's player character that he stepped on a puddle of blood and his feet got slippery, and will suffer a -2 penalty to hit and AC (boring) or tell him the the princess is being chased by an enemy and is climbing the mast. Now he has to deal with the bag guys, and rescue the princess (again)
There's no limit to the number of unlucky events (and luck chips) the DM can set, or more appropiately invent on the fly. It's his respnsibility to use them to improve the game.
There's however a limit to how many chips can a character hold, equal to one plus 1/5 of their level, rounding up. If they have the max number of chips, they can't gain new chips. That way, they'll spend them frequently, instead of hoarding them and being afraid of using them.