I would not recommend preparing for it. NWEA MAP should measure your DC's academic progress in reading, math, and possibly other subjects (depending on what your school uses it for), much like a ruler measures your child's height. The test is computer-adaptive, meaning that questions become harder or easier, depending upon the responses of the child. The first time your DC takes it is usually considered a baseline. Ideally, your child will "grow" by getting a higher RIT score each time he or she takes it.
While it is not perfect, I prefer MAP testing over other grade-level tests, since it can provide an indication that a child is working above grade-level (which can be confirmed by out of level testing).
HTH!
