Order two practice tests. Buy a similar test (the CogAT itself is not available to individuals). Check the level which you should purchase: it will depend on what grade your child is in.
Talk to your student about the format of the test and your expectations. Do not put too much pressure on your child. Remind her that this test measures natural abilities, not general knowledge. Describe the three different sections (math, verbal and nonverbal) to your student and chat about which areas she may be strong in, and which areas may be tougher.
Administer the first practice test in order to see which areas are stronger or weaker than others. Assess the results of the test, and employ exercises to strengthen your child's thinking skills in the appropriate area.
Practice analogies for the verbal portion of the test, patterns and relationships between numbers (+, -) for the math section, and study relationships between shapes and designs with your child for the nonverbal portion. Remind him that this test is a measure of natural cognitive ability, and doing these exercises simply help him to work carefully, methodically and logically, which can encourage strong thinking skills.
Have your child take a second practice test, so you can compare scores and help her become familiar with the types of questions she will see on test day. Encourage your child to work slowly and methodically, taking her time. Reassure her that when test day comes, it is okay to not know some of the answers.