How do I stop my clothes from shrinking?

Heat is the enemy. Most shrinkage comes from hot water and, especially, hot tumble drying, which relaxes and tightens the yarns — cotton and untreated wool shrink most. Wash cold, and either hang-dry or tumble on low or no heat, and you avoid the bulk of it; "pre-shrunk" fabric helps but rarely eliminates a final few percent. The care-label symbols exist precisely to tell you the safe temperatures, so read them before the first wash. Doing this protects the fit you paid for and extends the garment’s life, which is the quiet, unglamorous half of a good cost-per-wear result.