As someone said earlier, the first Boing is the most important. Make sure to have enough force on the first boing and to hit in the center of the string. From there it becomes a matter of making sure that all the axes are aligned, and rhythm.
The angle of yoyo vertically (Is it tilted?), angle of string vertically (Is the mount leaning towards or away from you?), angle of yoyo horizontally (Is the yoyo twisted in relation to the mount?), angle of string horizontally (Is the mount twisted in relation to the path of the yoyo?), and the PATH OF YOYO IN MOTION, all of these must be ONE line. Doing the trick eye level and looking down the string like a gun sight makes this easier.
I tend to think of the motion not so much as a "boing", which would be a constant bouncing, but a series of pulls and releases. These pulls don't neccessarilly have to be a constant speed, but do need to coincide with the specific points in the yoyo's travel. In short, you PULL until the yoyo hits the center string and RELEASE until it reaches the end of it's movement on the other side.
This is how I learned it.
Now that i can do it, though, I sort of lean everything back toward me and kind of let the yoyo "fall" back to me after each "boing".
One more note. When you do this trick slowly, the yoyo will twist ion the string. You have to turn the mount accordingly or the yoyo will pop out of the two strings.
You can see both of these here. http://www.youtube.com/v/41eqp4z149I&rel=0