I have resolved several bouts of sore hip flexors over the years by doing simple legs lifts while lying on my side. 3 sets of 20-25, adding weights if I manage to stick with the program for more than a week. In my case, it seems the underlying cause has always been a muscle imbalance causing the hip flexors to be pulled tight. No amount of stretching or massage ever helped, but the leg lifts showed nearly immediate results (less pain sitting or running after 2-3 sessions). The first time I had issues I spent a few months sitting uncomfortably (at work or in a car especially, no pain while cycling) and doing a lot of useless stretching. I was incredibly happy and relieved when I read somewhere about doing leg lifts as they were finally an answer to my pain.