Practically, we can control our own minds to an extent. For some people, the following technique works well: Think of a "clean" pleasurable image of a place you've been or an experience that you enjoyed. Concentrate on this image for a while with your eyes open or closed. Feel the feelings, see the picture and hear the sounds all around you. Now, every time an old image or fantasy comes up in your mind which you want to get rid of, simply replace it with this good image (see “NLP Swish technique” for more info).
Others suggest focusing on your breathing as you inhale and exhale deeply for a couple of minutes. This exercise has the power to divert our awareness from our thoughts as we focus on the breathing in our body. It is also relaxing and releases the tension and pressure built up by the lust we experience through the fantasies.
One person wrote on our forum:
You can't wrestle with a pig and not get dirty. The thoughts will come - accept that. It doesn't mean anything. Worrying about them or doing something with the express purpose of getting rid of them just won't work - you're wrestling with a pig. It’s like trying not to think about a green elephant, that will only make you think of a green elephant more. When the thoughts come, try to just acknowledge that they popped into your head, wish them a friendly 'shalom aleichem!' and then move on to try and do something else.
In the Chassidic literature (see Likutei Moharon 27:8) it is brought down that getting bad thoughts and fantasies is actually a zechus. They give a person the opportunity for teshuva and proper tikkun for past aveiros. So many people get down when they get these thoughts and feel bad about themselves. But these thoughts are there in order for a person to chase them away and merit true teshuva. The thoughts come to a person so that they should uplift them. Kabalistically, the thoughts have somewhat of a life of their own and actually WANT to be uplifted! So just realize that your current challenges are intended for you to attain teshuvah shleima. Knowing this can prevent unnecessary hindrances and should invigorate us for continued growth.
Teffilah is a very powerful tool in this struggle as well. When feeling under attack by lust, say: "Please Hashem, save me from lust! I want to love You, not flesh and blood". Or say; “Ribono Shel Olam, I know that it is my fault that I have these thoughts. I don't want them. Help me to distract myself to something else and leave them be.” Even short “foxhole” type prayers can work wonders like: “Hashem, Help me. I can’t do this alone.”
See our website www.guardyoureye.org and sign up to the chizuk e-mail lists, to learn many more great techniques on dealing with persistent fantasies and lustful thoughts.