Knitwear rarely wears out. It gets ruined - usually in the wash, occasionally on a hanger. The good news is that the habits that keep a jumper looking new take less effort than the ones that destroy it.
Wash it less than you think
Knitwear worn over a top barely touches your skin. Airing it overnight is often enough. When it does need washing, use a wool cycle or hand wash in cool water with a mild detergent - never biological, which breaks down natural fibres.
Never hang a heavy knit
Gravity stretches the shoulders into points that never come out. Fold knitwear and stack it. If you're short on shelf space, drape it over the bar of a hanger rather than putting it on the shoulders.
Dry flat, always
Squeeze water out gently - don't wring - then press the jumper between two towels and lay it flat, reshaping the cuffs and hem while it's damp. A tumble dryer will felt the fibres and shrink the whole thing by a size.
Pilling is normal, and fixable
Little bobbles form where fabric rubs: under the arms, along the sides, where a bag strap sits. They aren't a fault. A cheap fabric comb or a knitwear shaver takes them off in a few minutes, and the garment looks new again. Do it before washing, not after.
Store it properly in summer
Clean everything before it goes away - moths are drawn to traces of skin and food, not to the wool itself. Fold with a cedar block in a breathable cotton bag, never sealed plastic, which traps moisture.
Do this and a good knit outlasts several seasons of the cheaper ones you'd have replaced instead.