SQL & PL/SQL

I'm using mostly object tables/associative arrays in my package and it is using lots of server memorey so i want to use a different technicque to save momorey .
like using create global temporary table or nested Table Store as or creating table run time at the start and droping it in the end .
do have any suggestion for this ?
for this package i'm retriving data from different table into object tables and do calculation in memorey before i send data to front end for reporting using reference cursor .
give me your suggetion or if you know any example code for this type of process using different technique please let us know .
thank you very much
regards
shailen Patel