Well seiza is the stretch itself. My advice as always, push the discomfort zone but not to the point of agony. It also helps if you have good posture throughout. Keep your back straight.
Some great comedic sketches in Japan revolve around people being forced to sit in seiza during certain cermonies. Us pretentious white guy samurai wanna-bes are not the only ones suffering.
ps. racial sterotyping given in the sense of aforementioned comedic sketch.