In a tomato sauce recipe, how can I cut the acidity?
My secret weapon is onion. Caramelize the onions first. This creates a natural sweetness. Always use ripe tomatoes; if you cannot then use canned. The canned are made from ripe tomatoes and tend to be a very good substitute.
Also, the celery and carrot suggestions are very much a good addition - you are making a classic tomato sauce when you include the celery and carrots, 1 part each celery and carrot to 2 parts onion. Add the onions first to get the caramelization.
I don't get too fancy with herbs, usually one good fresh herb does it for me, basil or sage are two favorites of mine.
There is a suggestion to add cream, why not use the parmesan end you may have in your fridge, it is dairy and allowed to slowly simmer in the sauce will lend a very nice flavor note.
Acidic sauces are usually reserved for nights when I make a puttanesca sauce - it goes with the capers and calamata olives I add to it.
Don't forget to season with salt and pepper.
Yes, onion works great that way. Make sure to sauté them at low heat, see also http://cooking.stackexchange.com/questions/8289/why-do-onions-taste-sweeter-when-cooked-at-lower-temperature