Wool sweaters can have a scratchy feel for several reasons. Some wool types are finer than others; merino wool and cashmere are both very smooth, with fine fibers that are likely to be soft and itch-free. Lambswool, Icelandic wool and mohair may have a higher itch factor. Reading a sweater label before you purchase can help you avoid that entirely; but there are ways to reduce the itch from a sweater you already own.
Fill the washing machine with cool water, and add 1/2-cup of wool soak.
Leave the lid of the washer up and insert the itchy sweater. Push the sweater under the water, and allow it to soak for an hour.
Move the washer dial to the spin cycle and allow the barrel to spin and drain. When the cycle has finished, move the sweater.
Spread the sweater on a flat surface to dry. Depending on the size, thickness and type of wool your sweater is made from, it may take from one to three days to dry.
Fold the sweater and place it in the plastic bag. Zip the bag closed to secure.
Place the sweater in the freezer until shortly before you are ready to wear it, and return it to the freezer afterwards. Freezing smooths and sets the fibers, which reduces the amount of itchiness you'll feel when wearing the sweater.
This technique will help with a sweater that sheds as well, since it plumps up and softens the fibers. Use a large sink or tub in place of the washer if desired. You can soak multiple wool sweaters at the same time using this method.
Don't allow the washer to agitate; the sweater could felt or shrink. Don't use regular laundry soap to soak your wool; a conditioning, rinse free wool product is best for reducing the itchiness.