1. Remove the growing tip of the plant in early autumn, by cutting the main stem a couple of leaves above the top truss of tomatoes. This will channel the plant's energy into the remaining fruit, but it will also encourage more side shoots to grow, so you'll need to remove these regularly.
2. Remove any remaining flowers, as these will not have time to develop into fruit, and take energy from the plant.
3. Remove some of the leaves to allow more sunlight to the fruit. Eventually you may end up with a bare stem holding nothing but fruit, but don't worry - the tomatoes will still ripen on a bare stem.

These techniques should help, but we still seem to end up with a glut of green fruit as the first frosts of winter are looming. Previously, I have always just left these on a sunny windowsill to ripen. This year, I decided to run an experiment to try a couple of different methods to see which worked best.

The experiment ran from October 15th to October 21st, 2020.
 

The first step was to wash the tomatoes in cold water and dry them. All six of the fruits used in this experiment were Alicante tomatoes cropped on the same day, and none showed any initial signs of ripening.

The photos show from left to right:

1. Two tomatoes in an unsealed tub, kept on a sunny, south-facing windowsill
2. Two tomatoes with a banana in a sealed tub, kept on the same windowsill
3. Two tomatoes with a banana in a sealed tub, kept in a dark drawer in the kitchen

Day 1

Day 2

Day 3

Day 4

Day 5

Day 6

Day 7

The Science

Bananas (and apples) give off a gas called ethylene, which acts as a hormone associated with the ripening process. Trapping the ethylene gas with the tomatoes clearly works, but so does direct sunshine, and the combination of both worked best of all.

What I would do differently

Because I only had a couple of tomatoes in each container, and opened them daily to take photographs, the containers did not hold much moisture. However, if you try this for a large quantity of fruit in a sealed container, this becomes a problem. Here are some things to address if you have a lot of tomatoes to ripen:

1. Do not include any split or damaged tomatoes.
2. Space out the tomatoes so they are not touching one another, or the bananas.
3. Check them daily and remove any fruit that shows signs of rotting or mould.
4. Check for the build-up of moisture in the container and dry it off if needed. To prevent moisture completely, I'll be trying a tub of dehumidifier crystals, or moisture trap.