I haven't the courage to machine wash 100% wool myself. I wash it by hand in cool water, and, too cheap to buy Woolite, I just use whatever dish soap sits next to the sink. I learned a slightly different drying method than others here mention: I expand an adjustable window screen (I've also used an old baby gate, with large plastic mesh) so that it is supported by both sides of the bathtub, and lay the sweater (or socks, whatever) on the screen so that it can air dry from both top and bottom at the same time. My thickest wool sweater is usually dry after 24 hours of that.
Cleogrrl nailed it; unless you want to be surprised by doll sized wool clothing, avoid heat and agitation as much as possible. The opposite of this is intentionally shrinking and compacting the wool fibers into a dense felt, a process formally called "fulling", but some folks just call "felting". To achieve this, should it be your goal, wash the wool in alternatively boiling (or as hot as your hands can stand) and ice cold water, and use as much agitation and friction as your hands can muster. Your wool will become felt, and the final felt product will be significantly smaller than the wool with which you began. This shrinking can happen very quickly and surprise you if you're not paying close attention, so if you wish to full (or felt) wool down to a particular size, go very slowly and keep checking the fit as you go along. I'd therefore recommend practicing with something you don't care about before fulling something you consider important.