Simmer the sauce gently to reduce excess liquid
Add more grated Parmesan cheese
Whisk in a small amount of cornstarch slurry
Stir in a little flour-based roux
Add cream cheese for extra body
Use less milk, cream, or pasta water next time
Let the sauce rest for a few minutes off the heat
Toss in the pasta and let the starch help thicken it
Add a small amount of butter and whisk until emulsified
Cook the sauce a bit longer over low heat