Never throw away* a hand knitted item of clothing as you can unravel it and use the yarn to make other clothes, scraves, mittens, hats and knitted or crochet blankets.
Start by carefully unpicking the seams (a very small, sharp pair of needlework scissors helps with this), then start at any cast off edge of a piece and begin to unravel, winding as you go around an A4 size hardback book.
Fasten the skeins with a scrap of yarn, then gently wash by hand and hang out to dry. When dry, put back onto the book and wind into balls. Make a note of the weight for future reference.
*of course, it goes without saying that if you get the chance to pick up hand-knitted garments for pennies at jumble sales or charity shops then go for it! I bought the wool in the picture, when it was an actual jumper, for 90p and the resulting yarn weighs nearly 600grams. I admit I have not yet wound it on to a book and have made myself an extra job by ball-winding it first - but I find that easier!