I use the Puppia also but they have a vest style that is great. They step in it and then it has a velcro 3 inch circle and then above that at the double d ring it clips closed.
I used this on my sons Boston Terrier who is 95 percent muscle. LOL
and he could not make it come off or release the clip or velcro from his pulling.
I love it for my Yorkies because it is easy to put on, comfortable and durable. It will also not put a whole in your wallet.
Here is a website with them
http://www.spoiledsweetpets.com/puppiaharness.html