If the sun is coming directly through the windows, your air conditioner will be working overtime to cool the house. A simple way to use less power is just to block the sources of sunlight – you can try drawing heavy curtains across windows – but sometimes you need the light. In those cases, try to diminish the light instead.
Thin drapes are one way to do this, as is the use of two sheets of newspaper (assuming you don’t mind that look). Some people go for a compromise, such as clustering large leafy plants near direct sources of sunlight – this diminishes the light while jazzing up the place a bit. If you want a permanent solution, you can also tint the windows.