Summer is waning, and the days are getting shorter. September marks the time of year where diligent tomato care pays dividends in the shape of glossy, colorful tomato harvest. Most importantly, you really need to start pruning the plants, allowing almost-mature fruit to ripen and discarding any very small or grossly immature green fruits. This is especially true on plants that produce larger fruits. There is not enough time in the season/day to mature a big slicing tomato or a medium-sized paste. I know you don't want to, but remove all of those green tomatoes from the plant will allow the almost-mature fruits to ripen successfully. For notes on how to prune, read this post from earlier in the summer.
Secondly, I recommend getting aggressive about harvesting tomatoes. When fruit is nearly mature, it often times cracks. Cracked tomatoes are a product of fluctuating water levels for the plant. If the plant takes in too much water, it swells the fruits which may not have enough elasticity in their skin to stretch, so the fruits crack. To minimize this, harvest mature tomatoes immediately as they are ready. Letting them sit on the vine risks a late summer rain and leads to mushy fruits. Always try and harvest fruits after a few days of dry weather - they are the sweetest then, and won't be overly saturated with moisture.
Also, if you haven't already, please check out my new TOMATO eBook. It's $2.99 for 16 awesome tomato recipes, including a handful of preservation recipes. (It's the time to save some for winter!)