So, do fun, non-sexual things as a diversion (engaging in outdoor or fitness activities come to mind)... that way you'd be focusing on something physical and mental rather than sexual.
Err. While this is all pretty helpful, there is one problem... me and my girlfriend rarely get to spend some time. Alone...
I'm not trying to stop my hormones all together, only suppress them.
Personally, I think that your sexual desire is very natural and it would be a mistake if you tried to destroy the natural functioning of your hormones. In your situation I think that masturbation or other non-coital activities together with your girlfriend (oral, manual sex etc...) as previous posters suggested.are the most natural ways and they will not cause any hormonal imbalances or damage to you. If you want to abstain from sex, it is OK too, but you should probably learn to accept that the sexual desire which you feel is a natural thing and it is impossible to "kill" it in a harmless way. Playing with hormones and damaging their natural functions can be quite dangerous.
As for the last poster who speaks about "self-control" - well, this word has different meanings, but if it means fighting against the natural functions of your body or if it means surpressing natural and healthy functions, then I doubt that it is something very positive and healthy... on the contrary, in some cases it can cause more harm than good. (Sometimes the fire which is cooled by water leaves nothing too, just a mess and darkness if you put out the fire which might have been very helpful to you and bring some light and warmth into your life...)
As you see, self control is not viewed highly around here. A fire that is not banked and cooled burns brightly but quickly and leaves dead ashes. Are you in this for the relationship or for the gratification?
There's OTHER forms of sexual contact you two can engage in without actually having intercourse. Oral sex, mutual masturbation, etc...
It is generally not recommended to tinker with the hormonal balance. If you both are feeling attracted physically, you can do a lot of foreplay without penetration and later you can masturbate safely after you are satisfied. This will make your love more intense.