Finally, if you are still here it means that you properly applied point 1 to 3 of the checklist and you are still experiencing problems with permalink.
I have one last point that you should check out. Sometimes WordPress does not communicate properly with you database. You should check the setting in your WordPress Dashboard are properly stored in the WordPress database. So log in to your favorite database management tool (I use phpMyAdmin) and go to the WordPress database. Find the table ”wp_options” and check if option name ”permalink_structure” has the proper permalink specification.
Finally, find the option name ”rewrite_rules” and flash its option value (do not worry, WordPress will re-populate it). Sometimes the value is stuck and prevent a proper use of the rewrite option.
Hopefully this checklist helped you to solve you permalink problems.