A well-made tomato sauce has a fine balance of sweetness and acidity, derived from its tomatoes and other ingredients. That balance can vary with each cook's personal taste, but some sauces are inarguably too acidic and must be tweaked to mellow their flavors. There's a sizable body of kitchen lore for coping with the problem, but a few of those techniques are more practical and functional than the rest.
If dinner is almost ready but your sauce is unpleasantly acidic, you'll need solutions that work quickly. One is to physically reduce the sauce's acidity with a pinch of baking soda. Dissolve 1/8 to 1/4 teaspoon in cold water, and stir it into your sauce. There should be no discernible taste of soda, but the acidity will be blunted. If necessary, repeat the process. Both sweetness and salt can interfere with your brain's ability to process acidic flavors, and both can be used to tweak your sauce. A half-teaspoon of sugar or the equivalent liquid sweetener will make the acidity less obtrusive, or add a salty ingredient such as shredded Parmesan or Asiago cheese. As a last resort, add a pat of butter to the sauce. It coats your taste buds, and interferes with your ability to taste the acid.
Use a Soffritto
If you're making the sauce from scratch, one way to give it a mellower, less acidic flavor is to start with a soffritto. That's a mixture of onions, celery and finely shredded carrots, gently slow-cooked in a small amount of butter or olive oil right at the beginning of the cooking process. Use equal amounts of celery and carrot, and twice as much onion. These vegetables add rich, savory flavors to the sauce and they're all high in naturally occurring sugars. As the sauce simmers they break down and largely disappear, but their sweetness and flavor give the sauce a rounded taste and mask its acidity.
Peel Your Tomatoes
If you're making the sauce with fresh, whole tomatoes, take the extra time to peel them. You can cook with whole tomatoes and simply puree them to make the skins disappear, but much of the tomato's acidity is found in its skin. You can remove the skins with a sharp peeler, but professionals usually blanch them. Cut out the core of each tomato and cut a small "X" at the bottom, and drop them into boiling water for about 30 seconds. Transfer them to a bowl of ice water, and the skins should slide off easily once they're cool. Chop and seed the tomatoes, and use them in your sauce.
Upgrade Your Tomatoes
Your tomatoes themselves might sometimes be the problem. Unless you're using freshly picked, dead-ripe tomatoes, they might be acidic and astringent right from the start. Canned tomatoes can give a more reliable result, because they're picked and packed at full ripeness and don't need to be shipped cross-country for days. They're naturally sweeter and more flavorful, and will produce a sauce with less-assertive acidity. There is some variance between brands, so try a few until you find one that makes good sauce. Among chefs and food enthusiasts, Italian-grown San Marzano tomatoes are considered to have the finest flavor of all.