1. 10g now uses a new cost based optimizer by default
2. tkprof and explain plan help a ton you may also want to check out ADDM with its sql tuning advisors.
3. Don’t forget to run analyze command on your tables, if you do not have a good set of statistics the optimizer will often choose a bad path.
4. Take advantage of the AWR reports (similar to good old statspack) run a snapshot prior to and following your query for additional tuning information.
5. Be wary of any joins done on the v$ tables, the new cost optimizer does not always handle them well and you will need to add the use hints to get decent results. I have done an explain on some queries where I join two or three v$ tables and it results in a cartesian joins. Oracle has provided hints so these queries actually complete in a timely fashion, as opposed to the 45 minutes it was taking before.

I hope this helps you with some things to think about.

Keith