I found that I couldn't just sit seiza to practice sitting seiza. With nothing else going on, the discomfort I felt was all I thought about. Instead, I started doing little things in seiza (ironing clothes, reading a book). That way I could set small goals for myself - one more shirt, two more pages, etc.
Incidentally, I don't know if anyone can corroborate this, but I have been told in the past that part of the reason the seiza posture proliferated throughout feudal Japan was because it was painful and made moving difficult upon rising. For instance, if your responsibility were to protect your lord and a potential threat/enemy came to visit, by putting him in seiza you increased your chance of intervening should he attempt anything because you would be assured that he would be moving slower.
Food for thought, anyway
[Edited by Magma on August 28, 2000 at 02:48pm]