When DD took the CogAt, the only "prepping" I did was ask her to read slowly and take her time. I told her the first questions would be so easy, she would be tempted to fly through the test, but that's just what the sneaky test makers were betting on.
If she didn't read carefully it would be very easy to choose an incorrect answer without realizing it.
I told her it wasn't the goal to be the first one done, it was the goal to get as many questions right as she could.
She did very well on the CogAt, personally though, I dislike this test being used as entrance to gifted programs. I think it rewards perfectionism. I would hope the districts use additional measures in coordination with tests like CogAt and Naglieri. Although I guess it's much better than solely relying on teacher's subjective assessments.