There are a few simple ways to make it more difficult for herons to get to your fish.
Provide structure - simple and effective. Providing structure gives fish an easy escape route that they will use when they sense danger from above! Easy to install MossBack fish habitat structures are one effective method to quickly add structure.
Dye the water - Dyeing the water makes it more difficult for predators to spot fish under the surface. Adding dye at more than the standard dose for algae and weed control is even better for beating the heron at it's game! Black or dark blue pond dyes are especially effective and completely safe for all aquatic organisms.
Heron decoys can be effective when used at the right time of year. Herons tend to be solitary critters for most of the year and the presence of a decoy heron on your pond can deter others. The BIG exception to this is mating season. Get that sexy decoy put away for the mating season which in moderate to northern climates falls in the months of March to May. Southern mating seasons can range from November to April. It's important to know your local mating season to effectively use a decoy. If you choose to use a decoy, be sure to move it once a week or more. It doesn't take very long for a heron to figure out that a heron that doesn't move isn't really a heron!
For smaller ponds netting and other aerial obstructions to discourage the heron from landing can be effective.