Foot slappers usually have weak tibialis anterior muscles.
This muscle is the one to the front and slightly outside of your shin bone. It's the one that bulges up if you put your foot flat on the floor and then raise the ball of your foot whilst keeping your heel on the ground.
The classic way to strengthen this muscle is to sit on a table with your feet dangling and hook the handle of a medium size tin of paint over your toes and do some raises. Don't overdo it though because if you run with tired tibialis muscles you can get injured.
As well as controlling the front of your foot during the landing phase of running, the muscle has one other function - to lift your foot clear of the ground after it pushes off. If you take small strides you will be using the muscle much less. This may be why it's weak. Perhaps your stride has become shorter over the last couple of years? You might want to do some general leg stretches to lengthen your stride a bit.
I don't really get involved in discussions about the pros and cons of heel vs forefoot striking. It's all a bit unscientific.
