Here's my tip: Search Function. There is at least one thread either started or updated every other week on this subject. C/S, Rev J
Yoga. After a few weeks of yoga exercises you might be flexible enough to accomplish this. If not, then keep exercising until you gain the necessary flexibility.
I was recently able to get a bit in my mouth. It was the first time I had tried since I was young. I periodically stretched throughout the day. This was actually at my office (don't ask me why). Then after everyone had left. I got undressed and stretched some more. I sat in my chair bent over and pulled on the bottom the chair and pulled the object of my affection toward my mouth. I was able to get a 'lil in. But, I will say it was not comfortable and, in a very horny state, I could see some injury resulting from the desire to do the deed. My back was a bit sore for a while after. Good luck. I am somewhat flexible to begin with.