I had to salvage a disaster mitten yesterday and I was so amazed by the results I decided to use the same trick on all the others.
The trick? Putting a wet piece of cotton muslin over a mitten and then pressing the heck out of it with a hot iron. I didn't even bother getting out the ironing board because a thick towel on the top of the washing machine, while unglamourous, does a fine job.
The amazing thing is that the fabric changes so completely when you do this. Unlike when you use heat, moisture, and bonus agitation (and it felts), when it just has the heat and moisture the wool relaxes and eases out. The mittens actually get bigger, as shown in this Before (left) and After (right) shot:
thereby resolving the too-tight thumb issue I had with some pairs.
And the wonky seams I couldn't seem to get right on the three-piece mittens?
They look normal now.
And I am immensely pleased.