There are no magic pills I'm afraid.
But, some very good stretches include:
1) Couch stretch/bent leg lunge stretch
2) Cobra/seal stretch
One thing to note about Static stetching to improve flexibility/ROM is that it requires quite a bit more volume/time under tension than most people devote to it. If you are very tight I would suggest holding each stretch for anywhere from 90-120 seconds and doing at least 2 repetitions (per side if you are doing a unilateral stretch like the couch stretch). I would also recommend only doing this 1-2 times per week as connective tissue (like fascia) recovers at a slower rate than muscle tissue.
One other thing that actually made a significant noticeable difference for my hip flexors was a psoas "release."
To do it you will need a foam roller, a pillow (technically you don't but it makes it way more comfortable), some floor space and some time.
Place the foam roller on the floor so that one end is wedged against something sturdy (a wall, the base of a couch, etc...), then place the pillow on the other end of the roller so that it visions the end (you can go either lengthwise or widthwise depending on preference and size of your pillow).
Next lay down on your back with your head on the roller and the end of the roller approximately between your shoulder blades, legs straight out in front of you. Now just stay there, usually for a while especially when you first start (I've spent a half hour in that position on several occasions). Your first goal is going to be to get your lower back down on the floor with the legs still on the ground. At first your hips will likely want to externally rotate (toes will turn turn out) as that decreases the stretch on the psoas, let them. Eventually over the course of a few days/weeks your psoas will replease more and more and your hips will naturally rotate back to neutral (toes pointed straight up) with the legs straight, flat on the floor, and lower back flat on the floor.
This release is safe to do every day as well. It took me a little over a week of doing it every night before I had my back flat, legs flat, and toes pointed at the ceiling. The results were pretty dramatic in terms of "rib flare," ability to Posterior Pelvic tilt, abdominal activation, and glute contraction. Now I only do it periodically and instead do more traditional mobility/strength work to keep them limber and help to improve flexibility, but definitely feel that this release has made a huge difference in how efffective those exercises have become vs doing them before doing this release.
Try it out and let me know how it goes. If it's not clear let me know.