You know Porter's Five Forces; you should also use his five tests of good strategy

Deceptively simply -- and challenging -- tests to making good strategic choices: http://blogs.hbr.org/cs/2011/12/jim_collins_meet_michael_porte.html

p.s. The essential guide to Porter's work offered as a link in the post, as well as Jim Collins' latest, Great by Choice, are both excellent and useful reads.