It sounds like the spell involves working with a Saint, probably Saint Jude or Saint Expedite.
Those saints like to be rewarded with public accolade. You make a "deal" with them, like: "Saint Jude, if you heal my cancer, i'll put an ad in my local paper praising your glory and honoring your name at the first of every month so long as i am alive." You've probably seen these ads before.
I wouldn't use such a working for simple luck. There are easier things to do to improve your luck that won't enlist bartering with saints, although praying to them is always fine.