For low psi, our psychologist recommended 'overlearning' in a particular subject as the best way to get faster at something. My dh asked about working to improve processing speed through other mental exercises, and would that carry over. The dr. indicated that this would not be the case. So, for math fact speed, study math facts, for word retrieval, study words. But this might only work if your child will put up with rote memorization exercises. We have looked to games which educate for some of this. Our ds8 hates the drudgery of anything that feels like studying. (Gee, I don't know where he gets that.)
Also, learning style can play a role in better retention/retrieval. Just last week he was to memorize the student creed at the new tae kwon do studio, and I was repeating it to him and asking him to repeat each sentence...slow going. Finally I just put the paper in front of him, he read it a couple times himself, and now he has it. Basically, once he sees something (eg: spelling words) he pretty well has it and can remember it. But auditory memory? Relatively weaker.
Put another way, visually he can learn something with 1-2 repetitions. Auditory learning seems to require a more normal number of repetitions 6-8.
Here is an interesting page I just found on learning style, figuring out your preferred learning style, etc.
http://www4.ncsu.edu/unity/lockers/users/f/felder/public/Learning_Styles.html
