As you are referring to the natural flex of the fork, i would suggest fitting a heavier duty fork (larger cross section) might help. For example if you are using 100mm x 40MM (2500Kg at 600mm) at the moment you could change these for 100mm x 50mm (3500Kg at 600mm) which would reduce the flex you experience. Of course there are many other fork sizes available, just pick one to suit your application.