Simmer longer to reduce and concentrate
Make a slurry: mix equal parts cornstarch and cold water, then stir into simmering soup
Add a roux: cook equal parts flour and fat, then whisk into hot soup
Whisk in mashed potatoes or potato flakes
Blend part of the soup, then stir back in
Stir in cooked rice or pasta and simmer until thick
Add pureed beans (white beans, chickpeas, or lentils)
Use a cream or dairy thickener: stir in cream, milk, or yogurt off the heat
Add cheese: stir in grated cheese off the heat until melted
Use egg yolk tempering: whisk yolk with a little hot soup, then return to pot and stir gently
Use tomato paste for thicker body in tomato-based soups
Add breadcrumbs or crushed crackers gradually, simmering to desired thickness
Add oats (quick oats or blended oats) and simmer until thick
Use flour directly only as a last resort: whisk flour with a little cold water first, then add and cook through
Season to taste after thickening (salt and pepper often need adjustment)