Q: How do you keep down the amount of spam comments you receive on your wordpress blog ?

A: First, I use the following options in the discussion panel:

An administrator must always approve the comment

Users must be registered and logged in to comment

Comment author must fill out name and e-mail

Additonally, I have installed and configured the following plugins:

Akismet
– Holds spam comments in their own queue.

Register Plus – Enabled with captcha keeps down robot registrations that create fake accounts to post spam comments.

Hope this helps you out!