I used to do a vinyasa flow class once a week. Kept me very limber. The flow yoga isn't about holding one position for a lengthy time, you keep moving through the positions for like 90% of the class. You'll have a puddle of sweat on the mat when you're done. no heated room, just very hard stuff to do. You'll be surprised at the level of exertion and soreness associated with flow yoga... It's not for sissies.
It was actually also great for building definition for me. The first two months I would be incredibly sore in odd muscle groups, sides of the bicep, hip abductors, side of the back nearing the small ribs. after about 2 months of it I started noticing a nicer looking waist line and that V cut at the hips became very noticeable. Better looking biceps and triceps too. No real increase in strength or in measured size, just made the muscle, for lack of a better term, pop. Think this has something to do with supporting muscle groups being developed from new types of stress and the lengthening effect on the whole muscle. One day I'll actually look up the reason.
The other stretching programs mentioned have really piqued my interest as well. You could incorporate the yoga or stretching on a rest day or if you really like it and get into the advanced stuff, restructure your whole schedule for it.
I also stretch a little before workout, sometimes between sets 2 and 3 (just a 5 second) for legs specifically, and always after.
I'm running 4-Andro/Epi-Andro and mk right now. I've been MUCH stiffer and almost pulled something a few weeks ago because I was neglecting the stretch.