When I went to take the PL/SQL exam I had done the following.

-- Taken a class on PL/SQL at a community college
--Read about 700pp on PL/SQL
-- written about 35 to 40 scripts
--taken the STS program until i scored 100% on all 296 questions --

I scored about 85% on the test, but I felt that the way Oracle worded the test was very tricky. A solid foundation of all of the things on the DBA Checklist on the OCP pages at oracle would be most crucial to passing the test.

I agree with CBW.. STS is no indication on how well you will do on the real exam.. unfortunately.. but it is a good way to know whether or not you are at least prepared to take it. If you are scoring poorly in STS, I would suggest going over the DBA checklist and make sure you feel comfortable with all of that.