I have a pyrenees mix who is an escape artist. Standard collars and harnesses were not effective. He has not as of yet slipped this harness, though I don't recommend leaving any such dog unsupervised on a lead.
As a public service side note, he runs off because he is sexually intact. I had another pyrenees mix that was neutered by the vet's timeframe, 6 months, and this is now known to cause curvature of the long bones and raise the likelihood of bone cancer and the need for TPLO plates. This was the case with my beloved dog, who only lived to be 6. Please, if you have a giant breed, do not neuter until they are done growing.