Tips for keeping beak in check..

I was just wondering if anyone had any tips or tricks to keep your little torts beaks in check? Ours has got a little over grown, and I've tried to take him to the Vet on three occasions to have it trimmed.. each time he would get worried as soon as we go in the building (particularly if a dog visiting the same vet barked) and go in his shell meaning they can't do anything about it!

We even left him there for a day with his favorite food etc but still apparently he didn't come out enough for them to be able to do it. The cheeky bugger though as soon as we leave the Vets would poke his head out and be happy again!

I've tried cuttlefish and he has rocks / pebbles but he's just not interested in them so I'm wondering what you guys too to keep it to a reasonable level.