When the cold weather starts to break make sure you add a water conditioner to any water you add from the hose. Some people add chloramine to tap water, which never spreads throughout the pond, and which also provides an excellent food source for algae.
Adding barley straw to a pond at the beginning of the season is also wise. It comes in bail, pellet, or liquid form. As barley straw breaks down, it creates enzymes that help to keep water clear.
Phosphate removers, whether in liquid or pellet form, will also help fight algae in ponds. Phosphate is one of the main nutrients that algae use for food, so removing excess phosphates helps to kill algae.
There are also products on the market that dye pond water blue. These liquids are usually added once a month, and are not only pleasing in appearance, but also effective at preventing algae. The blue dye in the water reflects sunlight so that it does not penetrate the surface of the water, which prevents algae from turning the sunlight into the energy it needs to spread throughout your pond.