Patience is the #1 ingredient.
Pure vitamin E oil is said to help but never proven.
However, once scar has healed/closed, keeping it moist is said to help (this is how vitamin E oil might help as would many other scarreduction creams.
But basically, you have to give it time.
From what I read, during scar formation, you get a lot fo activity and collagen in the scar area so you have a lot of scar tissue. Once the scar has closed, there is a process of contraction that happens to reduce the scar's width (bringing the 2 edges of the cut closer together) and at same time, it appears some enzymes are generated to reduce the amout of collagen and this reduce the size of the scar. (not sure about the specifics but this is the gist of it). This happens over a period of months.