Cursing In the Books

[Inziladun]

Well, not cursing, as in orcish vulgarities, but curses, as in wishes of ill will from one person to another.

We see many examples of this throughout the books. Sometimes they work:

But this I will add: he that loosed the shaft shall break his bow and his arrows and lay them at my son's feet; and he shall never take arrow nor bear bow again. If he does, he shall die by it. That curse I lay on him'.

Mîm the Dwarf to Túrin's companion Andróg, who apparently died of arrow-wounds on Amon Rûdh.

Sometimes they don't work.

'By the beard of Durin! I wish I had Gandalf here! Curse him for his choice of you! May his beard wither!'

Thorin Oakenshield

What I'm wondering is this: what exactly makes a curse work? I'm not speaking of Morgoth's curse of Túrin, as that curse was accomplished through the active work of Morgoth. But in the case of Mîm, he did not act directly to make the curse come true: it just happened.
Now, Mîm's curse might seem to be just, as his son was shot by Andróg as he fled from the outlaws.
Thorin's would seem to be unjust, because it was bourne of greed and anger, that Bilbo had given the Arkenstone to Bard as a bargaining chip.
Who brings about the events laid out in a curse? Who decides what curses are just, and which are not?
Isildur, though he certainly was wronged by the Men of Dunharrow when they broke their oath to him, was a mortal Man. He had no power to hold the souls of the Oathbreakers to the earth after their lives ended. So who did it? Who is the judge of whose curse ought to be fulfilled, and whose should not?