Even thought I am not a fan of the IUD at all at this point, have you considered the Paraguard ? We are in the same boat as you. We are sure we are done but not 100% positive. We have been using NFP since 3/07 and not having any issues. I track my cycles on http://www.mymonthlycycles.com
It took me a few months after Mirena to have my cycles back to normal. They tell me when I will be fertile and I increase it by 2 days before and 2-3 days after. Just to be safe :) We use condoms during fertile times only. Sometimes I avoid intercourse all together during that week. So far so good !