If the arms of your sweater are long enough for a monkey, or maybe the whole thing is a size too large, you might be able to get it back down to size. The following techniques work best with wool sweaters.
Fill the sink with warm water. Soak the sweater in the water for 10 to 15 minutes.
Take the sweater out of the water and roll it in a towel to remove as much moisture as possible.
Spread the sweater on a towel placed on a flat surface, scrunching the sweater into the size and shape you want.
Return to the sweater every several hours, rescrunching it as necessary, until the sweater is dry.
If the above steps haven't fixed the problem, you can try tumbling the sweater in the dryer on low heat, checking it every few minutes to see if it's the right size. If you're not careful, however, you can end up ruining the sweater, and at best drying it will likely weaken the fibers.
To avoid stretching out your sweaters, store them flat rather than on hangers.