Simmer the sauce uncovered to reduce excess liquid
Add tomato paste
Stir in a cornstarch slurry
Mix in a flour slurry
Add grated Parmesan cheese
Blend in mashed tomatoes
Puree some of the sauce
Add a small amount of breadcrumbs
Stir in a roux
Add cooked and mashed vegetables
Mix in cream cheese or ricotta
Add ground meat and cook longer
Use less water or broth next time
Let the sauce rest to thicken after cooking