We have been in touch with someone who has succeeded already for close to one hundred days, to stay clean from viewing improper pictures and from masturbation. He is a true hero and real spiritual soldier. Recently, he decided to try and guard his eyes and mind as well, but he was having trouble with this and he wrote us as follows:
Q. I've failed in guarding myself from mental stimulation. I have not been able to control my eyes properly. I keep fantasizing, fantasizing about whoever on the street is dressed immodestly. I would like to masturbate again, but I'm beyond that; I'm not going to screw up after so many years of effort. I refuse to go back to that again.
A. Dear spiritual soldier! Don't be discouraged. You should know that guarding the eyes and the mind is even more difficult even than stopping to masturbate and view p-orn. It is a constant work. Every time you go out into the street, and especially when you are feeling emotionally weak, it is very difficult to keep the right frame of mind to garner the strength necessary for guarding the eyes properly. Once a person has seen that they can live without masturbation and p-orn, and once they have recognized it as an addiction, they can learn the techniques and with effort and prayer they can keep the strength not to go back to it. But guarding the eyes is a constant and continuous struggle and requires a lot of patience. You will have many ups and downs along the way. Don't expect results right away. If you succeed in turning away from something arousing even a few times a day, it is already a great accomplishment - even if you failed many other times that day. And don't worry, you will start to see progress over the months and over the years, as long as you never give up hope. And never stop praying to G-d. When you see beauty or feel lust, cry out "G-d! I love you, not flesh and blood!"
See also here for more Chizuk on dealing with sexual fantasies...