Having been in chastity off and on for the last 15 years, I've learned a thing or two, mostly the hard way (pun intended).
First, girth. I used to have similar problems as described, and agree that more lubricant when putting it on helps. Ice/cold applied to the penis, and putting it on a bit after orgasm also helps. The trick is to get as much blood OUT of your member as possible. Once the cage is in place, a properly fitted chastity device will keep you from getting hard.
Second, "pushing and pulling". Also previously mentioned, use a smaller ring and smaller spacer. These help keep your penis from getting as hard, and give it less room to maneuver. The CB-x style devices also have an 'anti-pullout' addition you can purchase, as well as various other implements of destruction to... "discourage" Willy from trying to wake up.
I did a post giving some advice to a first time chastity dabbler over in KT... might be helpful, or not...
http://www.kinktalk.com/talk/showthr...110#post104110
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Scrap D Flasher
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