The quality of the morph is highly related to the number of control points.
The more points, as long as they are accurate, the better the registration.
Interestingly enough I found that more points were needed than I might have expected, but still a reasonable amount.
And it's also very important to focus on certain areas of the image.
For instance I was doing the morph with faces. Originally I thought that a single point in each eye would be sufficient.
I later found that it took at least 3 points to get the eyes to match up in a realistic way.
I was also surprised at how important it was to match up girls hair, their hair required a fair bit more work than I expected.