Simmer the sauce longer to reduce excess liquid
Add more grated Parmesan cheese
Whisk in a small amount of cornstarch slurry
Stir in a little flour slurry
Add cream cheese for extra thickness
Mix in a beurre manié
Use less milk, cream, or broth next time
Let the sauce cool slightly before serving
Add a small amount of mascarpone
Incorporate an egg yolk off the heat
Blend in a little mashed cauliflower
Add a small amount of instant potato flakes