Drive a lower emission car if possible. Electric cars are exempt from the London Congestion Charge. If you're only visiting for the day, consider parking outside the zone and using public transport for the rest of your journey.
That means you won't need to pay the fee if you drive into a congestion zone in an electric car, so that can save some real money if you regularly drive in the city. Plug-in hybrid vehicles (PHEVs) are not exempt from the charges, and you'll have to pay if you drive in congestion charge zones.