Simple. He could adopt Ron Paul's policy of reviewing ANY legislation that comes his way and comparing it with the Constitution. If it doesn't meet Constitutional muster, he would (like Congressman Paul) issue a press release outlining the reasons he won't sign the bill into law. He could do that with any legislation that Congress sends him. Does he? Nope.
Also, he could adopt the same policy for legislation that he, himself proposes. If it's not Constitutional, it gets scrapped.
That would be a start in the right direction.