What is the proper form of address for a former governor?

Former governors are not 'officially' addressed as Governor (name)because there is only one Governor of a state at a time ... and doing do is not respectful of the state's current governor.

This holds true for other offices where there is a single office holder ... The Speaker of the House ... the Mayor of a City .... the Chief Justice of the Supreme Court.

WHEREAS there are many senators, admirals, judges, ambassadors at a time ... so calling a former office holder by one of those titles is not in conflict with a current office holder.

So most correctly he is addressed by his highest, non-exclusive honorific:

Senator (Name) a man who served as Governor twice ...

When Dwight Eisenhower left the presidency he went back to General Eisenhower.

That's also what Albert Gore has done ... he's back to Senator Gore, because he can't be Mr. Vice President.

Bill Clinton and George W. Bush don't have a title to go back to so they are both "Mr."

... and Colin Powell is not longer Mr. Secretary or Secretary Powell ... He's back to General Powell.