I would talk to a Dr. before doing anything, to make sure you don't have an injury, but I've found general ankle and foot strength goes a long way to help with seiza. A program like this has helped me out a bit to strengthen my ankles (http://www.csuchico.edu/~sbarker/inj...nkle_rehab.pdf
). I really like the towel scrunches. Something else I like to do is stand on one leg and swing a weight around in random directions or do furibo (VERY large bokuto in Jikishinkage-ryu) cuts on one leg. This really strengthens things up, too. Mix this with sitting in seiza for short periods of time throughout the day to strech things out and it might help a bit. Before standing from seiza try coming up on your toes to bend them back a little and strech them out, too. Good luck!