Pat Kirtland, posting on the Knitting Beyond The Hebrides email group.
Here's her suggestion, quoted from her email:
"I would reblock the finished sweater to the size you want....and when it is dry hold a steam iron over the top of the sweater about an inch away from the sweater, this will "set" the yarn."
I had never heard of setting the yarn before, but I will certainly give it a try.
In the past I'd always blocked individual pieces first, in order to make joining the pieces easier. I'll probably think twice about doing that again. Perhaps it is better to block sweaters after they are seamed.