The hippocampus, part of the limbic system, and lies next to the medial temporal lobe. It is made up of two structures, the Ammon's Horn, and the Dentate gyrus, each containing different types of cells. The memories are stored by augmentation. Augmentation occurs when a signal passes across the same synaptic path many times. Each time the signal passes, it augments the synapse so the next time the same signal will pass more easily. This occurs since neurotransmitters that are stored in the neurons increase suitably to allow faster and stronger signal conduction. This is why childhood memories are generally stable and retained above others. Aside from that ... err... So, if you keep thinking of someone, the memory of that person gets augmented.