My hip flexors have felt tight from time to time. I do a stretch for them several times per week. When they were really tight I did it every day, along with a bunch of other gentle stretches for my legs and hips, and I tried to walk every day at the park. It took a few weeks, but the tightness did ease up. Hot epsom salt baths really help, too! Use lots of salt....at least 2 cups per bath! It's relaxing and supposedly the salt helps the muscles to feel better.
I want to add....GENTLE stretching, don't overdo it. I only do 2 sets of 30 second "holds" for these things. Sometimes being overzealous on the stretching can create more problems than it solves.
Also, some people think that it's better to stretch after you're warmed up. So maybe go for a 15 minute walk first or something like that.