At the end of the season, or as a result of late tomato blight, you may find yourself surrounded by green tomatoes. After some experimentation I have found that the key to ripening green tomatoes is to wrap them in newspaper.
First, try to pick your tomatoes after the skin has become shiny - this will increase the chances that your green tomato will ripen. Next, discard or cook any green tomatoes that are starting to rot. They won't ripen and there are lots of great recipes for green tomatoes (see link below).
Separate your green tomatoes into two groups: green tomatoes that that have some color and those tomatoes that are still completely green.
Wrap each tomato individually in newspaper keeping the two groups separate. Once wrapped in paper, I keep each group in a separate bowl or in a brown paper grocery bag.
The reason to keep the totally green tomatoes separate from those that have started to ripen is that tomatoes that have some color will ripen much more quickly than totally green tomatoes.
Check the group of tomatoes that has shown some ripeness every day or two. Check the group of totally green tomatoes, once or twice a week, shifting them over to the partially ripe group, once they start showing color.
For a yummy green tomato treat, try this recipe for crispy fried green tomatoes.