Simmer the sauce uncovered to reduce excess liquid
Add tomato paste and cook it in
Stir in a cornstarch slurry
Mix in a flour slurry
Add grated cheese such as Parmesan
Blend in a small amount of cooked vegetables
Purée part of the sauce
Add a roux
Stir in cream or mascarpone
Add breadcrumbs
Simmer with a lid off after adding pasta water sparingly
Use less liquid next time
Let the sauce rest to thicken slightly before serving