I really don't think the RIAS would be terribly prep-able. The verbal subtests look at the ability to make connections and draw inferences among verbal prompts, so, while a good vocabulary helps, prep that is not "g" dependent, such as memorizing vocabulary words, is probably not going to have a dramatic impact. The non-verbal items likewise require reasoning about similarities, differences, and relationships between and among visual items.
I fail to see why anyone would want to spend the time and money to prep their child so they could get a deliberately inaccurate evaluation of their child's intellectual profile, but I know that there are a lot of parents who seem to think that getting an artificially inflated score and using it to put their children in placements that are too hard is somehow doing their kids a favor. They don't realize that they are not only doing their own children a disservice, but they are potentially harming other children who scored similarly but without prep, who really would thrive in such placements, but who might be denied entrance because of the programs' previous experience with children who were placed based on similar but prepped scores.
Descriptions of the RIAS subtests can be found
here, starting on page 101.
