Also start small and work your way up. Starting too large will tear. Best advice is to use something small and get used to penetration then start working up. There are a lot of nerves and it can be intense either pain or pleasure or both. The key is to relax the muscles. Yes pushing back helps. Not so hard like your goong to the bathroom though. That'll push it out and can cause damage as its pushed it. Hope you enjoy it. It can be extremly pleasurable.
N1X: as long as its done in stages and your Mistress pays attention you should be fine. For your own knowledge research it though. You can never have too much knowledge.
http://www.healthcommunities.com/ana...th/index.shtml
See if this helps.

Best of luck to you both on your journeys.