First, the good news is that aside from being unsightly, green algae doesn't pose a threat to your fish.
Some people add one of many types of algae-eating fish to their tanks -- though you'd have to take into consideration your current stocking level.
Aside from that, algae thrives on two things, light and nutrients. If the aquarium gets any direct sunlight, that can contribute greatly to the algae problem. Limiting artificial light can help keep algae under control as well.
As far as nutrients go, more frequent water changes and keeping nitrates low may also help.
Again, though, aside from being unsightly, algae isn't a health threat to the fish, and there isn't an aquarium keeper out there who hasn't had algae in his/her tank from time to time.
"I wish, I wish, I wish I were a fish" --Henry Limpet