levite

When I broke up with the girl I was with the longest (5 1/2 years), I also had a hard time not thinking of her when I was tossing off. Two things that helped:
1) Look at a lot of porn. I got very into watching/looking at homemade porn [other peoples', not mine] online at this point-- a taste I still retain-- and seeing non-professional, real women getting nasty really helped cleanse the mental palate.
2) Find other hotties you know personally to perv on. At that time, I worked with a fiercely hot chick, who also happened to be married. I knew I couldn't make a play for her, so it was safe for me to fantasize about her on an ongoing basis. I made her a regular feature in my sexual fantasies, and that also helped.

Other than that, only time will help. I still very occasionally fantasize about my exes, and it's good now.

__________________
Dull sublunary lovers love,
Whose soul is sense, cannot admit
Absence, because it doth remove
That thing which elemented it.

(From "A Valediction: Forbidding Mourning" by John Donne)