This can be really hard. There was a time between 10 and 15 years ago that you could find many erudite discussion on astrology, but that time has faded for the moment. Now everyone believes that they can learn everything they need to know on the INTERNET. This has dumbed astrology down to some lowest common denominator since people aren't willing to buy books and study. There are still some real forums but the moderators or owners who used to be actively involved now rarely say anything, or there is no moderation and people are just plain mean to each other. I used to own one but no one came to talk and after awhile I gave up.
These are some that I know about.
1. The Tyl Forum. Noel Tyl rarely says anything except for occasional vocational astrology: http://noeltyl.com/discussion/index.php#2
2. AstroDienst. This is a German site with zero moderation. People attack for no reason, and people here like their astrology very very simple: http://forum.astro.com/cgi/forum.cgi?cid=pjafilehFJzk5-u1315356625
3. Bob Marks Forum: Best at the moment. People are kind of nice, but there is almost no moderation. Possibly Bob Marks has died: http://www.bobmarksastrologer.net/forums/index.php?sid=8b3e59a0531e6f212cf37393b92f8070