Agree with fold, don't hang. And don't wash every time you wear unless soiled or sweaty.

To get wrinkles out I use a steamer or steam with the iron while sweater is hanging.

I hand wash the higher quality sweaters and dry flat. I have had bad experiences with the gentle cycle on my washer. The lower quality sweaters and the "unknown" brands get dry cleaned because I'm worried about shrinkage. I have read that it's better not to dry clean because the solvents are not good for the cashmere fibers... don't know if that's true though.

Old cashmere sweaters can be felted and then used to make toy animals or cute little fabric boxes if you're crafty. Same for old wool sweaters. So if you end up with a holey faded sweater at some point don't just dump it.