Rob Bell, in his book "Velvet Elvis", says:
The ancient rabbis had all sorts of things to say about this passage, but one of the most fascinating things they picked up on is the part about God's back. They argued that in the original Hebrew language, the word back should be understood as a euphemism for "where I just was".
It is as if God is saying, "The best you're going to do, the most you are capable of, is seeing where I ... just ... was."
That's the closest you are going to get.