Focus on the desires behind the lustful thoughts. 'What is it that my heart desires from this?'
A good book, with guided prayer and reflections in each chapter, is <em>Be Restored</em> by Bob Schuchts . You might also check out the podcast he hosts with Jake Khym called <em>Restore the Glory</em>. They did a several episode series back in September on sexual struggles and discerning the wounds & needs behind them.