SPI_saveplan stores a plan prepared by SPI_prepare in safe memory protected from freeing by SPI_finish or the transaction manager.
In the current version of Postgres there is no ability to store prepared plans in the system catalog and fetch them from there for execution. This will be implemented in future versions. As an alternative, there is the ability to reuse prepared plans in the consequent invocations of your procedure in the current session. Use SPI_execp to execute this saved plan.
SPI_saveplan saves a passed plan (prepared by SPI_prepare) in memory protected from freeing by SPI_finish and by the transaction manager and returns a pointer to the saved plan. You may save the pointer returned in a local variable. Always check if this pointer is NULL or not either when preparing a plan or using an already prepared plan in SPI_execp (see below).
Note: If one of the objects (a relation, function, etc.) referenced by the prepared plan is dropped during your session (by your backend or another process) then the results of SPI_execp for this plan will be unpredictable.