Sex drive in a long term relationship

My boyfriend and I, when we first started dating, used to have sex almost every time we saw each other. Now, over a year later, we hardly do anymore. We live together and I just don't have the sex drive that I used to. I still love him and definitely still think he's sexy, I just am never in the mood anymore. I've heard this is a common occurrence with long term relationships. Does anyone know why this happens or what might help get us back in the love making game?