The secret to hitting anything reliably is consistancy. If you vary your approach angle, altitude, and/or your speed, you'll also see variance in impact points. It takes a bit more effort to pay attention to the small details, but it also leads to more consistant results (success).
I'd recommend reading the Dive Bombing tutorial at the AH Trainers site for starters.
http://trainers.hitechcreations.com/ It's a great place to begin.
With a little practice you'll be hitting ground targets with ease. I use to love D-Bombing in an F4U. Line up on the target, and fire all my rockets (.salvo 8), quickly switch to bombs, drop them, and pull out. FH gone in one pass, every time...
For a small target (GV's) I would try a steeper dive, and don't forget to lead them if they're moving.