The supermarket dilemma: organic green beans from Kenya, or ordinary green beans grown just down the road?

From an environmental perspective, organic farming is usually considered the better option, experts say. It's less intensive and relies on ecological processes rather than synthetic fertilizers and pesticides.

But some experts argue that those benefits can be outweighed by the energy used to fly food to the supermarket.

"I feel really conflicted on this issue every time I shop," said New Urbanism's Kunz. "I want to do the best thing for the planet and buy local, but I also want to do the best thing for my body and buy organic."

Alex Randall is a a sustainability expert from the Centre for Alternative Technology in Wales, U.K. "We need to look at everything we eat and try to replace high environmental-impact foods with lower ones," Randall said.

"So avoiding foods that have travelled a long way, or are out of season, is a good place to start," Randall said.

But this doesn't mean you have to avoid all foods from afar.

Foods that have a longer shelf life, such as cereals and wine, can be transported by ship rather than air. "Shipping is better for the environment than air-freighting, but if you can local products are the best of all," said Randall.

(Learn how to spot misleading labels at the store.)

"For the greenest meal, shoppers should look for local organic produce direct from farmers and [neighborhood] markets," said Friends of the Earth's Atkins.

"Avoiding supermarkets cuts out the middleman and helps to ensure farmers get a fair price for the produce they sell."

For a while, reusable bags were the must-have accessory: In 2007 many shoppers sported their Anya Hindmarch-designed "I'm Not A Plastic Bag" carrier. But does spurning plastic bags really make that much difference?

For one, plastics as a whole are an increasingly common type of waste: In 2007 in the U.S., about 31 million tons, or 12.1 percent of total municipal waste, was plastic. Much of that waste makes it into the world's oceans, where it can harm wildlife.

(Related story in National Geographic News: "PHOTOS: Giant Ocean-Trash Vortex Documented--A First.")

Many cities, and even some countries, are phasing out or taxing plastic bags. Ireland introduced its plastic bag tax in 2002, charging 15 Euro cents (22 U.S. cents) on each bag.

In 2007 San Francisco became the first U.S. city to introduce a plastic-bag ban, and now Seattle is considering following suit. (Take a plastics quiz.)

But there are downsides to getting rid of plastic bags. Plastic-bag manufacturers claim that there has been a huge spike in sales of bin liners and garbage bags, since people no longer have the free ones from the grocery stores.

Paper bags are also increasing in popularity. Some studies suggest that paper bags require more energy to manufacture and release more greenhouse gases when degrading than plastic bags. Friends of the Earth's Atkins suggests that we should concentrate more on what goes into the bag than what the bag is made of.

"Plastic bags are only a tiny part of the problem. To avoid sending materials to landfill or to be incinerated—which is a polluting and expensive process—we should avoid producing waste in the first place by buying only what is necessary, and reusing and mending our possessions."

Pay a small "green" supplement to your electricity provider, and in return they will provide you with "zero emissions" electricity from renewable sources such as solar and wind—or so it seems. Can you leave the air-conditioning on and watch your plasma screen TV, assured that your electricity is all squeaky clean? (How green is your flat-screen?)

Victoria University's Vale said people have to change their energy consumption habits, too. "The only answer is to start by using less power," he said.

"Only when you have really reduced demand can you then do good by using green power. Otherwise you are just expanding the total electricity generating capacity. Less really is more."

And not all green-energy tariffs are created equal. Some companies promise to install as much renewable energy capacity as their green tariff customers use, while others buy offsets to compensate for polluting sources of electricity.

"In general, buying your electricity from the companies that produce their own renewable energy will have a greater impact," said the Centre for Alternative Technology's Randall.

As well as increasing demand for renewable energy, green-energy tariffs can make a statement to society, Randall said.

"If lots of people opt for green-energy tariffs, it sends a powerful signal to government and industry."