Since I was just doing this yesterday, I thought I would post some pictures:
While this is specific to the BB shell, it applies to lugs as well. I made some heavy mandrels (?) that fit common socket sizes and use them to bend the sockets and then act as an internal anvil for reshaping the lug sockets:
Adjusting the chainstay socket:
by https://www.flickr.com/photos/54319503@N05/, on Flickr
That usually opens up gaps around the perimeter:
by https://www.flickr.com/photos/54319503@N05/, on Flickr

Then I use a brass punch to close them up using the solid bar inside the socket as an anvil:
by https://www.flickr.com/photos/54319503@N05/, on Flickr

That usually leaves small divots in the shoreline/socket:
by https://www.flickr.com/photos/54319503@N05/, on Flickr

So then, I re-shape the shoreline and socket to get a nice fit:
by https://www.flickr.com/photos/54319503@N05/, on Flickr

Final fit up:
by https://www.flickr.com/photos/54319503@N05/, on Flickr