By using Proloquo2Go's VocaPriority levels (Primary, Secondary and Storage), you can organize the vocabulary by frequency of use. Buttons that are frequently used should be placed in the Primary level, while buttons that are used occasionally should be placed in the Secondary level. Buttons that are infrequently used should be placed in Storage.
Storage is not visible to the user, but contains all the buttons that are not used by the user or for which the user is not yet ready. As a user becomes more proficient, you can promote buttons out of storage to the appropriate level.
For exact instructions on how to use VocaPriority, please see this tutorial.