Well, just getting past the first 15 minutes is the hardest part; after that, your dick begins to lose sensation. I think about other things. I think about what I am doing with her, or I look up at the crucifix above my bed if I'm doing her doggie style, or I think about how the relationship will end. Different positions help keep it exciting enough that I don't lose an erection. I look at h!er body and that's a turn-on to keep it going.
Key thing is not listed to all things she's saying, like "You make me cum! You make me cum!" or "I really love your cock!" She wants me cum so she's going to try to get me into that fantasy-world of hers. Key is don't believe it, keep a serious face, do your things...kiss her, caress her body, but don't forget that this relationship is likely to fall apart...keep that in mind and it balances out the good thoughts.
Be Stoic about things--Seneca reminds us that sex is just the rubbing together of parts until ejaculation. Makes it sound kind of stupid, doesn't it? It is kind of silly, except for the procreation part.