My advice for getting more speed and quickness in your left hand is to force yourself to do things with your left hand that you would normally do with your right hand. For example, try leading on the hi-hat with your left hand and place a ride on the left when practicing. Your left hand will only get faster as the muscles are developed to compensate and match those in your right. From there it's just a matter of practicing until you get the desired results. Aside from that, the advice mentioned in the other replies work good also. On a slightly different note, I had the same challenge when I wanted to go from playing flat-footed to heel-up. The only way I made the transition to where I can now use both the flat-footed and heel-up technique equally, was to force myself to play heel-up no matter what I want playing, ie. slow, fast, medium. After the muscles developed in my calves and I was able to control the pedal effectively, I saw results. Again, as the other poster commented, it may take a while depending on how long and frequently you work at it.