Wool doesn't need washing as often as you think. Airing a jumper overnight solves most of what a wash would — and every wash shortens its life.
When you do wash: cold water, wool-specific detergent, and either a hand wash or a machine wool cycle at 400 spin. Never wring it. Roll it in a towel to press the water out, then dry flat.
Hanging a wet jumper is the single fastest way to ruin its shape. Flat, always.