I've always been thinking about hyperextension but never got round to researching on it. My teacher always encouraged keeping heels together in first position..and I thought she was being fussy in the past until I became a teacher...and it all makes sense now! I've been controlling my own minimal hyperextension quite successfully, and corrected students with more pronounced hyperextensions.
I read an article by Lisa Howell recently and that was only when I got more concerned about my student's hyperextensions. Since there're so many articles online already..I think I can sit back and let you guys do the clicking =P
Dance Teacher Magazine
Wandering Apricot's Blog
Deborah Vogel's Dancing Smart Newsletter
Dance.net -- Little Moomoo's post (has useful tips on feet exercises as well)