Simmer uncovered to reduce excess liquid
Stir frequently to prevent sticking and burning
Add tomato paste and simmer 10–20 minutes
Add a cornstarch slurry (1–2 tbsp cornstarch mixed with 2–4 tbsp cold water), then simmer until thickened
Add a flour slurry (1–2 tbsp flour mixed with equal cold water), then simmer to cook out flour taste
Whisk in a small amount of roux (equal parts butter and flour), then simmer
Stir in grated Parmesan and simmer briefly
Add a spoonful of butter to enrich and slightly thicken
Blend a portion of the sauce and stir back in
Use less liquid if starting from scratch (reduce broth/water)
Add cooked, finely chopped vegetables (mushrooms, onions, peppers) and simmer
Add ground meat and simmer longer to reduce and thicken
Use bread crumbs in small amounts; simmer to thicken
Use instant potato flakes (1–2 tbsp, simmer until dissolved)
Add Greek yogurt or cream off-heat for a richer texture (then simmer gently, not boiling)
If too thick, loosen with a splash of water, broth, or reserved pasta water and simmer briefly