One of the most frustrating disasters to befall a beloved piece of clothing is the phenomenon of wool shrinkage. Favourite sweaters can shrink when exposed to wet and warmth at the same time, making them unwearable. Fortunately, the process of wool shrinkage is not as irreversible as it is commonly assumed. A careful process of washing and restretching can restore your sweater to it's original size, sparing you the need to replace it.
Prepare a cool, dry room. Close windows and blinds during sunny weather, or wait until night. Run fans or turn up the air conditioner.
Soak the sweater. Be sure to use the coolest water you can comfortably hold your hands in (you'll need to handle your sweater to stretch it, so this is important). Hold the sweater under water long enough to make sure it's fully soaked (until bubbles stop rising to the surface of the water).
Wring the sweater out by hand. Do this gently to avoid damaging the fibres--if your sweater is delicate, you may want to squeeze out the water by pressing and folding, rather than twisting the fabric.
Hang the sweater in the prepared room. Stretch out a dry towel beneath where it's hanging to catch any drops of water that may still fall.
Aim fans to blow on the sweater. Wait a few hours.
Check the sweater's progress. The body of the sweater will have stretched out some, as will the arms on a long-sleeved sweater. Use your hands to stretch out any sections that haven't yet stretched sufficiently, especially parts like cuffs or sleeves that won't have been weighted down.
Lay the sweater on a flat surface, on top of a spread out towel. If it needs any further stretching, use hand weights to stretch the material out and hold it in place. Let dry.