Three steps for honing SQL skills first.. SQL is the foundation :)
Answer the questions asked in the forums -- begin with simple sql statements. (By this way atleast u will make an attempt to provide solution after u solve them)
Even if someone answers it already, try to provide alternate best solution (By this way u will try to think different)
If someone contradicts you, provide justification if your argument is valid (by this way u will check the documentation of oracle)
I would encourage you to participate this forum on a regular base.
Also, for a daily PL/SQL related challange, register @ http://www.plsqlchallenge.com
It is founded and maintained by 'PL/SQL Evangelist' Steven Feuerstein.
Another useful article: