1) A good beneficial decree that will come upon a person, even against his will, and no bad will come from it. On the contrary, the Sages explain that such a decree will even uplift a person.
2) A bad decree that has been written and sealed in Heaven; there is no rescuer.
3) A decree that comes according to a person's requests and efforts.
Regarding the above, the Pele Yoetz advises:
"In light of this, a person must take into account that he is from the third category. And everything that is in his power to do, he shall do."
And "efforts" include so much. When you express gratitude for 100 different things (or even just 20 different things), you are changing yourself from within and re-making yourself into a vessel for blessing and good decrees.
When you give tzedakah, your are changing your spiritual form.
When you decide to do any good deed, you are changing your destiny.
You can go from being a treif-eater to a Jew who only eats kosher. You become a different person inside, and that affects the Heavenly decrees pertaining to you.
When you remain silent in the face of an insult, when you hold back the lashon hara exploding in your mouth and heart, when you say "thank you!" instead of "how dare you!" -- you are making yourself a receptacle for blessing and sweet decrees.
In the above, the Pele Yoetz placed his delineation of decrees in the section on meriting one's soulmate.
But really, the concept applies to so much more.
May we all merit to become vessels of sweet decrees and blessings.