Time, really. That's the best answer for me, and the only one I personally have any use for. I've also found that doing your best to block that person from your thoughts, to not think about them, not look at pictures, not walk around thinking of all the little things that remind you of them eventually yields results.
For me, the first is something that will almost always help you, and the second is something that will help you only if you are desperately committed to getting past that person.
Then again, I'm not the expert. Hell, I'm still new at this being alone thing myself. These are just solutions I've found that worked/are working well enough for me to feel like a human being again.