A previous poster mentioned airspeed, and he's right. Most of the
students I fly with have trouble nailing an airpseed and holding it.
For power-off approaches, use the best glide speed and then make
the approach just a bit high, applying flap as necessary to steepen
the glide to hit the spot. You can also reduce the approach speed a
bit to steepen a glide, but watch that you don't start such a serious
sink rate that you end up short without any flare speed. Reducing
glide speed 5 knots can make a big difference in the glide angle.
Diving at the spot is the wrong thing to do, as it increases airspeed
which will only cause float when you reach the surface.
I once read of the British training their recon pilots in spot
landings during the big war. They buried a 2x6 flush with the grass,
flat side up, and the pilots learned to touch down ON the board, in
that 5 1/2 inches, without bouncing and at a given airspeed. If they
could do it, so can we. We just don't care enough to get good at it.
Or, perhaps, we can't afford to get good at it.

Dan