there is a nice little plugin "confirm you are not a spammer" which cuts down on comments quite a bit. I also make sure my blacklist is up to date ( I use this one):

https://github.com/splorp/wordpress-comment-blacklist/blob/m...

with both of those solutions, my main site does about 5k UV's a day, and I see about 20-30 spam comments per day (as opposed to a few hundred, easy)