First, Marcella is the bomb, and I use her cookbook regularly. Here's her recipe: Marcella Hazan's Bolognese Sauce.

Now for my modifications:

• I double the onion, carrot, and celery servings.
• I am very liberal with my butter and olive oil as well - please use real butter and olive oil. I prefer to cook with unsalted butter, too.
• I buy whole San Marzano tomatoes instead of crushed tomatoes. I put 4 cans (because I double the recipe) into a bowl and chop them up with one of the can's lids or you can hand crush them. I add an extra can that I do not crush for additional flavor. 
• I only cook with good wine. Think about it: why would you cook with it if you would not drink it. My preference is Louis Jadot Pouilly-Fuisse. You can use red wine, but I have found that my sauce is sweeter, and the flavor is perfect with white.
• I never cook with milk. I always use heavy cream as it increases the flavor and simply tastes better.
• Since I double my recipe to freeze sauce for future meals, I use a pound of ground steak instead of ground beef and then add a pound of ground pork. I understand this means it's not traditional Bolognese, and that's okay with me. It really adds to the flavor, and I prefer it.
• I use less salt because the biggest mistake you can make is to oversalt a sauce, and since I freeze it, I prefer to add salt when I am heating it as it enriches the flavor. Eat your frozen sauce within two months for the best flavor.
• I also add Aidells Caramelized Onion Chicken Meatballs to my sauce because my son loves it. If you want to include homemade meatballs, do so. Just do not put them into your sauce. It should be added to the top of your pasta dish.