It's going to be a hot weekend, so I thought I'd quickly go over how much trees can affect the cooling of your home. The Canadian Federal government has a lot to say about how trees can help in the winter, but I'll cover that another time. For now, we'll use US tree authority sources.
According to the USDA Forest Service, trees properly placed around buildings can reduce air conditioning needs by 30 percent. The US Dept of Agriculture cites that the net cooling effect of a young, healthy tree is equivalent to ten room-size air conditioners operating 20 hours a day. (And on top of it, those same trees are absorbing the CO2 that our homes emit in the first place).
If you are thinking of planting some trees for shading/cooling purposes, think Canadian. Planting deciduous trees allows sunlight through the winter months, increasing solar energy; in the summer months, those leaves will block the heat and sunlight. But this kind of strategic planting requires planning. Considerations like the orientation of your house to the sun (obviously the south side of the house gets the most sun) and the kind of species you plant need to be thought of carefully. But if you take the time to draw out a plan, you can seriously save on your energy bills.