From my experience, once you have clover, the only way to rid yourself of it is to dig it up, plant new seed and grow a new lawn. On the plus side, clover is a legume and therefore Nitrogen is being fixed into the soil as I type this message. Also, if your lawn is a high traffic area, clover is very resistant to foot traffic and other traffic as well. It's a very hardy plant.
Maybe Scott can offer some other advice.