very sore hip flexors after a new workout--help?

... suggest ways to stretch it? The workout was straight leg raises, oblique sit-ups on a roman chair, push-ups on a big yoga ball, and this thing where I started in a plank on my toes and elbows, twisted a bit to straighten one arm, then straightened ...

http://ask.metafilter.com/247255/very-sore-hip-flexors-after-a-new-workout-help