Try a pick from the following handy methods for dealing with
unskillful mind states:
1. Lust is an attachment to an idea of a person, or certain aspects
of that person. Instead of thinking about what's under that
person's clothes, think about what's under that person's skin
2. Ajahn Chah once advised a young monk who was pining for his
girlfriend to have her send something of hers ... a jar of
faeces
3. Think about the untold suffering that attachments and
relationships bring.
4. Focus on this person's less desirable features. Imperfections.
Annoying habits.
5. To paraphrase a lewd story I once heard: theoretical girlfriends
are like Juliet. Real girlfriends fart in bed. Alter gender to fit
cirucmstances.
6. Lust, like all compounded things, is impermanent. Ride it out.
Replace it with something else. "This too will pass".
Guaranteed.
Now, don't take this too far. You're just trying to weaken a
delusional attachment to someone by showing it to be a delusion.
You're not attached to a person but to an idea of a person.
Best of luck.
