well there has to be lots of foreplay - teasing, tickling, massaging all parts of my gf's body before penetration. i like to enter slowly, because this enhances the pleasure for both of us. i am large enough that it would be uncomfortable for her otherwise.
the overwhelming thing in my mind is how incredibly intimate a thing it is for my gf to share her body with me so completely. it is not so much a 'conquest' or a 'dominance' thing for me, but i know it is a real turn-on for her to be so filled up and so vulnerable with someone she can trust. having slowly entered her until i am fully inside, it is hugely erotic to just stay still within her, with both of us savouring the sensation. there is nothing to beat the feeling of my cock being enveloped so completely in a such a warm and soft embrace, but with a hint of being gently 'gripped'.
if we are horny enough, the slightest rhythmic movement at this stage can be enough to get both of us off, though i usally manage to contain myself, so she can enjoy more orgasms from different positions, before i finally lose it.