Hi Subbu,
Follow the below activities.
1.Delete the old requests in PSA.
2.In DSO level delete unwanted data in changelog table.
3.Delete unused Aggregates.
4.In Cube level,Compressingthe databasedonnoof days as per project requirement.
5.In Reporting level deletingorrefreshing the Cache memory.
http://scn.sap.com/docs/DOC-39735
Regards,
V Srinivasan