Simmer the sauce longer over low heat
Add more grated Parmesan cheese
Whisk in a small amount of cornstarch slurry
Stir in a little flour slurry
Add cream cheese
Mix in a beaten egg yolk off the heat
Use less pasta water or milk
Add a small amount of heavy cream and reduce it
Stir in a butter-flour roux
Let the sauce rest for a few minutes before serving