Ripening depends on plants having sufficient light and warmth, slow ripening at the end of summer might be helped by reducing shading.
Gradually de-leafing tomatoes growing under glass will improve the amount of sunlight and air getting to the fruits.
But on no account take off any leaves above ripe fruit.
The number of trusses being grown can affect ripening.
Generally the more trusses that are allowed to form creates the situation where the uppermost trusses are competing for nutrients (potash) meaning they take longer to ripen.
Restricting the number of trusses will alleviate this problem, do this by removing the growing tip from the plant just above the last truss you wish to form.
Placing green tomatoes in a dark drawer will cause them to ripen off the plant.
Some people have been known to place a banana in the drawer to encourage the ripening process.
Check the contents of the drawer regularly, removing any ripe or rotting tomatoes as necessary.