I"ve been a faithful disciple of Limyaael since late 2003, and I've found while most of her advice is for fantasy writers, there's a lot on how to build realistic characters that don't annoy the reader over a longer story (both main and secondary) which could benefit almost any writer. NOt to mention, they're just entertaining to read. :)

http://coyotecult.com/communities/sfandf_c...es/limyaael.php

(I'd link the LJ where she usually posts, but this is more easy to see the full list and all links lead to the LJ)