Well, you know what you gotta do, so keep at it! The butterfly stretch is always a good one, and maybe the best one for BJJ and guard work. One thing I like to do is a variation of it where I put my legs in that position, but in a way where I can touch the ground with the knees, and go forward, with my hands on my back. I don't know if what I'm doing is very correct, but it helped a lot with my flexibility and the butterfly stretch for me. Putting my foot in my chest on that same position also helps me. Too bad I don't think I can find any pictures right now for you to get the idea...
Warming up is very good, especially if you'll stretch in the morning. I usually take the morning sessions and without a very light run before warmup I can't even touch my toes. After that, it's all good.