A perfectly vine-ripened tomato is one of summer's most memorable treats, with its perfect balance of sweetness, acidity and lightly savory notes. Reproducing that balance in a tomato sauce can be challenging, because cooking brings up the acidity in tomatoes as it does with other fruit. Some cooks add sugar or other sweeteners to their sauce, to tame the tomatoes' acidic bite, and it's a good technique if used sparingly. If you overdo the sugar though, your sauce will be too sweet and will require some corrective measures.
When you've taken any flavoring ingredient that one step too far, the simplest and most effective remedy is to add volume. It's a logical response: If there's too much sugar for your batch of sauce, increase the amount of sauce until you restore the balance. In the case of tomato sauce, that means adding crushed tomatoes or pureed whole canned tomatoes. If you're working with fresh tomatoes, chop a few extra and cook them until soft in a separate saucepan. Mash them for a chunky sauce or puree them for a smooth sauce, and add them to the main batch. Taste the result and repeat the dose if your sauce is still sweet.
The second alternative, either alone or in conjunction with the first, is to balance the sweetness with added acidity. A pinch or two of flavorless cream of tartar, or the citric acid crystals sold to prevent fruit from browning, is often all you'll need. If you have neither of those powdered alternatives, add a modest splash of lemon juice or mild vinegar, and stir it in thoroughly. Taste and repeat, as you would with lemonade, until the sauce's flavors are bright and balanced.