Wool shrinks with a combination of heat and agitation. Avoid those two things and you should be fine.

If a wool garment is one I consider "less delicate," such as a wool blend or a superwash wool, I will machine wash it in cold water on the gentle/delicate cycle and let it air dry. If the garment is 100% wool or one that I care especially for, I will hand wash it in the sink, again in cool water, and allow to air dry.

Just avoid hot water, and never ever ever put it in the dryer!