A good beginning strategy would be to, whenever possible, buy from stores whose business it is to sell environmentally sound products. One great example is The Body Shop.
Read a short article about The Body Shop

For most of our consumer decisions, we simply do not have access to all of the possible information that we might want to make a truly informed decision. However, that doesn't mean that we should not think about the environmental effects of our decision at all.

For example, if you go to the store to buy some snacks, you don't have any immediate access to information about how they were made, and the retailer most likely won't either. However, there are many things you can look for:

#1 is packaging. Some estimates say that over 1/3 of the waste we produce is from product packaging. Less is better, and paper based is better than plastic, styrofoam, or shiny aluminim-type packaging. Beware of unnecessary double-packaging! Also, look to see if the packaging includes recycled products, or if it is recyclable/reusable. (For example, yogourt dishes are great for storing left-over food or many other things!) For some information on the types of packaging to avoid if at all possible, click here.
Read about 5 Bad Packages

Other things to look at include where the product was made. If it's a local product, less transportation was required to get it to the store shelf.
More Shopping Tips

However, major purchases and decisions about using many services aren't made so quickly and give consumers the opportunity to gather information. Often, we don't think of environmental issues, but we should.

First, we should look at the most obvious environmental effects of the products we are considering. More and more often, specific options or products are available which are better for the environment than base models, though these are also often more expensive. Finding out about these differences is important though, as it can give you an idea of features to look for or compare when comparison shopping.

Next, go out and do your homework! Virtually all major companies have websites that you can search through to find out more about their environmental policies. They sometimes have detail about manufacturing processes etc which you may be able to evaluate with a critical, environmentally-aware eye. A company which has no mention of environmental policy may or may not have things to hide, but personally, I would prefer to do business with a company who is upfront and public about their commitment to protecting the earth.

There are also organizations that you can consult to help you make more environmentally-sound decisions. There are many online, and many that you could contact with just a couple phone calls. I'm sure that they would be so happy to hear that you are concerned about such issues that they would do whatever they could to assist you!

This still isn't all that you can do! If you find a product or service or company that you think isn't doing quite as much as they could to protect the earth's future, the first thing you can do it not buy it. You can also encourage your friends and family not to buy it. You can write letters to companies, asking them to have a better look at their environmental policies.