Clearing tables questions

Hi,

I have two fairly simple questions (I think) but I can't
seem to find the answer in the docs or mailing-lists.

I'm using pg_dump and pg_restore to backup my dbase.
However, I noticed that when I do a pg_restore, the
dumped contents are added to the currents ones.
This is not what I want. The tables should be cleared first.
How do I do this initializing the dbase?

Another  question I have is the following.
What sql command can I use to clear (delete) all tables not
starting with 'pg_'.
I can easily come up with a query that gives me all tables with
the specified prefix. But when I try to 'loop' using a cursor
I run into all kind of syntax problems.

I'd be very happy if you can help me with any of these questions.

thanx in advance,

Joep.