At its best:
Well into September, outdoor tomato plants will continue to produce new flowers, even though there is no real chance of them ripening naturally.
Your tomato plants should have formed three or four trusses of fruits by early autumn, so to ensure these all ripen, remove the top of each plant. Simply cut through the main stem a couple of leaves above the uppermost truss of green fruits.
Removing the top section of your plants may sound drastic, but it will encourage them to put all their efforts into producing ripe tomatoes. There is no point in them producing new fruits that have no hope of reaching maturity outdoors.