Cut down on your food air miles. Don’t buy any food that you know has been imported by plane (apart from fair trade* products like coffee, wine or sugar).
It’s best to buy seasonal fruits & vegetables as they are least likely to have been imported from the other side of the world. Be sure to scan the labels for information on where the food product is from. Try to avoid foods with many ingredients, too. It’s always better to support local producers, and to cut out as many ‘middle men’ as possible.
* Fair trade (two words) refers to the concept, which promotes the better treatment of small-scale farmers and farm workers from developing countries, esp. in Africa and South America. Fairtrade (one word) is the trademark of the organisation that issues certification to farmers and companies that meets its criteria. For a debate on how beneficial this really is to farmers and farm workers, read this blog post.