When I went through AFF I used to lay on my living room floor on my belly and rest my toes on the edge of the couch. I'd pop into neutral position, hold for five seconds, and relax. That helped I think.
In terms of losing good body position when you relax-- keep in mind that a neutral belly position isn't the same thing as going limp. You have to work a bit to achieve and maintain neutral position. You should feel air on the front of your legs and you'll have to push against it a bit to keep them in the correct position. Same goes for arms.
Honestly though, just keep working at it. Progress might feel slow but remember that you've only been working on this for short minutes at a time. You'll get there if you keep pushing!