Since I keep getting asked this question I've now added a permanent link to the drawing that Patrick of Yoga Garden in Yokohama drew up for me showing the various yoga poses he recommends for helping people get into full or half lotus posture. It's right there next to this article and it will be forever accessible via a link in the links section over to your left.
Although I do yoga myself, I didn't really personally use these poses to be able to get into full lotus. For reasons I cannot understand, I've always been able to get my legs into the full lotus posture, even back in the days when the rest of my body was so stiff I couldn't even touch my toes. The only problem was I couldn't hold the posture for more than a couple minutes. What I did was just hold the full lotus posture as long as I could and then switch to half lotus. After about six months of doing this twice a day every day I could hold the full lotus for a full 40 minutes.
But that's my story, not yours. These postures have helped yoga people get into the lotus posture for a few thousand years. So maybe you can benefit from them.