There are no Thumb Rules, that we just follow them as standards. As other person replied, we need to analyze SQl's. If you have SQLs that you are getting from sessions. Just make sure you have indexes on most used SQL joins. If not watch the sessions (V$session,V$SQL etc.,) for day or two . Find the SQL's that are used.

There are so many links on metalink.oracle.com that you can search.