Loose weight (unless you are already really thin) and practice flexibility.
The best I could ever do was to get the tip in my mouth by laying in the bed swinging my legs up. when I was about to cum I could take in the head and beat off into my mouth.
You really can't give yourself a great blow job. Just like it is really hard to tickle yourself, the body doesn't respond to self touch the way it does to tough of another.
But it is nice when it works to swallow the whole load of cum and just roll over in bed and savor the taste while you go to ....... Rather than having to wash up. I wish I could do it now.