Two things are important. As DC said, be careful not to burn the onion and garlic. The onions will carmelize and the garlic will become bitter. NOT good. Also, put your canned tomatoes through some kind of strainer to get the seeds out. Seeds will make your sauce bitter.

Most people simply add browned meatballs and sausage. However, the meat you add can make a difference in the taste of the gravy. If you like your sauce spicy, use hot sausage. If you like it sweet, add a browned chicken leg or a piece of pork (perhaps a country style rib) to your gravy. If you like it thick, cook it with the cover mostly off. If you like it thin, cover it.

I never add my sugar until the end. Depending on how you cooked it, you may not need it, so I always wait.