Some people cut off the trusses and place them in a sunnier / warmer place, like a sunny window sill, or you could try putting them amoung some bananas. The ethylene that bananas produce will help them to ripen.
If all fails you could try your hand at making some green tomatoe chutney. Good Luck Tbird+
Along with the other advice, I'd offer that, if you're expecting a frost, pull the entire plant up (roots and all) and hang it upside down in a sunny, frost protected area. I use a south facing garage window. They will ripen naturally and have that keen "vine-ripened" taste. We've had such tomatoes a month after a killing frost...
Please pick them as soon as possible as tomato blight is once again spreading across the country and this will kill them off almost overnight if the virus lands on your tomato vines. I have my green tomatoes resting on sheets of newspaper on trays on my lounge windowsill where they catch the sun, and they are ripening pretty quickly.
Thank you all very much for your useful advice, I have decided to try each of the suggestions (as I have enough of them to do so) and will post a report on my findings in due course. Thanks again.
1 cups plain white corn meal
1 cup plain white flour
1 1/2 tablespoon salt
Pinch of black pepper
1/2 cup cooking oil
Combine flour and corn meal. Wash the tomatoes and pat dry. Cut tomatoes in 1/4-inch slices. Sprinkle with salt and pepper. Dip each slice in corn meal-flour mixture and lay aside on waxed paper. Heat oil. Fry tomato slices until golden brown. Drain on paper towels. Serve hot.
This recipe is very adaptable. Some prefer using all flour or all corn meal. Some people prefer *green* tomatoes, others prefer slightly pink tomatoes. Try serving with ranch dressing or sprinkling with garlic powder before cooking.