Great question — answering as the UphillTech team:
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...
Full answer by Christina Tenney: https://cooking.stackexchange.com/a/4122 (CC BY-SA 2.5).
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