Most of my base layers and midlayers are now merino, so I often have some wool to wash. My rules:
1. Use a non-detergent soap - Atsko Sport-Wash, Woolite, Ivory Flakes have all worked well for me for many years. 2. Use a front-loading washer or wash by hand, as the agitator on a top-loader can injure the fabric. On anything really expensive (like cashmere) I always wash by hand. Soak for 20-30 minutes, rinse, and then press excess water out; don't wring it out.
3. Air dry flat. Using a hanger can stretch the fabric. Never use a dryer.