Probably the best stretches for hip flexors that I know are lunges. Make sure to do them properly though!!! Your back knee should be on the ground, and your front foot should be far enough in front that your KNEE DOES NOT GO INFRONT OF YOUR FOOT (that's the most common mistake I've seen). Once in the lunge, sit up so your weight is in your hips and not in your front foot and push your hips toward the floor. Make sure that you are staying centered and not leaning to the side. It also helps to rotate between splits instead of doing one, bringing your legs in, then switching to the next one. That sounds confusing I'm sure, just get in a left (or right leg) split, then rotate directly into a middle split, hold it, then rotate to the last split. This helps stretch the flexors (and hamstrings) get used to rotating for stretches to bows and rotating from scales to stretches also. Hope this helps!!!