Simmer the sauce uncovered to let excess liquid evaporate
Add tomato paste
Stir in a cornstarch slurry
Mix in a flour slurry
Add grated Parmesan cheese
Blend in cooked vegetables
Add a small amount of mashed potatoes
Stir in breadcrumbs
Use less water or broth next time
Add a roux
Puree part of the sauce
Add cream cheese
Simmer with a lid off until reduced
Add ground meat or sausage
Use less canned tomato juice or watery tomatoes