I think partner stretching is the way to go - I used to be OK in flexibility and within a few months I could do the splits both ways. Do your basic stretches (say, hamstring stretch) and have someone push down on your back gently and consistently. Breathe and relax the whole time (relaxing is the key). Release and relax, and then stretch again but this time have your partnet gently push you further. Make sure you do a proper warm up before this and just keep at it.