Beurre Manie
Mix together equal parts softened butter and flour until it looks like thick butter and all the flour is incorporated.
Roux
Melt 2 Tb. butter in a pan, add 2 Tb. flour, whisk together and cook until lightly browned
Cornstarch Slurry
2 parts cold water mixed with 1 part cornstarch
Whisk any of these into your hot liquid, stirring until thickened. Don't over-cook the cornstarch slurry or it will start to break down and become thin again.