similar questions answered earlier in this forum, please go through those also.. you can find some info in the following link..http://www.ibmmainframeforum.com/system-programming/topic475.html
Apart from the above, I have done performance tuning in my previous project which i am listing below..
1. Build indexes on tables whereever necessary if you are using search fields
2. Optimize the access path - Check with your DBA
3. look for unnecessary fetching of data by any query
4. WHERE clause in the queries is the place where you can improve performance a lot by giving appropriate values
5. Check for unnecessary loops related to data fetching
6. Check for data integrity
7. improper coding also sometimes cause for performance issues.. so code optimization is needed by a senior person
Hope this info helps...