Before washing, I always remove the tags and cut the sweaters apart. This includes cutting open the the arms.
This is how the sweaters look after that process is complete.
I usually end up washing the sweaters with like colours, which is why they are piled this way. In this case I washed the sweaters for one wash cycle on hot and than threw them in the drier on heavy duty o pull off any extra lint. I think I threw a few tennis balls in the (front loading) washing machine too. It worked great and the sweaters were felted, but not too stiff.
So, what does one do with felted sweaters? I've been checking out Resweater for some great ideas. She also sells sweaters in bulk here if you don't have the inclination to troll for sweaters yourself. Her prices are pretty reasonable too.
For reference, I only buy my sweaters on 50% off days. It takes me about 40 minutes to drive each way to the store and I spend about an hour there. Each sweater is usually less than $10 - 50%, so usually less than $5 per sweater, but rarely less than $3. Kris sells bulk sweaters for as low as $4 per sweater when you buy them in a bulk set of 10, and you don't have to do any of the work!
I recently finished a great project using felted sweaters. I can't wait to share it, but it is on its way to it's recipient, so I don't want to ruin the surprise. Be sure to stay tuned!