In my opinion, the curses that come true are the ones that are fair and have meaning behind them. Sometimes people in ME cursed without really meaning what they say, or at least in the full extend of what they say. For example, when Mablung found Turin near Haudh-en-Elleth, Turin said something along the lines of "A curse on your errand! And a curse on Menegroth! May winter wither it!" No matter how touchy Turin was on the subject of Doriath, I don't think he really meant that much harm by his words.
Other curses are more "well-reasoned", and the person who said the curse knows that he really wants it to come true.
As for who decides which curse will come true, maybe none do, but "real" curses come true by themselves. I've noticed that many people, especially those of noble descent, tend to have some inner power in them. Maybe their curses are more likely to come true? And even a common person can have such power, it's just that descent helps
. When Morgoth cursed Hurin, he said that the shadow of his will will follow his faily wherever they go - so maybe he's not the only one able to do things with his will?...