Sometimes when a person tries to guard his eyes the evil inclination tells him "why are you not letting yourself experience the beauty that G-d created in his world? There is so much incredible beauty out there, why do you try to ignore it?" One must realize though that "there is beauty and there is beauty". Some of the beauty in this world was put here just to test us, to trick us, and to turn us away from G-d. From this type of beauty we need to keep far away. But sometimes to have the strength to do this, we need to focus on the other beauty, the beauty of G-d. We need to learn to see the beauty in G-d's handiwork, and focus on that instead when we are being tested. For example, each one of us ourselves is such a beautiful expression of G-d's handiwork, the amazing perfection of how our body, mind and soul all work together. Also, how much beauty there is in the world around us! The beauty in our children, in our own wife, in the sunset, and in the perfect collaboration in nature - all designed by G-d to work in perfect sync and to be pleasing to man...
And if we indeed work on ourselves and focus on G-d's infinite beauty instead, then truly there is nothing more beautiful in G-d's eyes than this.