>>3Sometimes, talking with her throughout the act on what you like, and asking her what she likes can really help. Also, don't forget foreplay!
There's also the fact that some women have a really low libido (naturally or because of drugs). If she's on drugs, having her quit, if possible, is your only option. If she has a low libido, you're fucked. She'll probably end up saying she's "unhappy" with your relationship and fuck other guys, though.