I didn‘t really care what my beret looked like through BMQ, SQ and BIQ. I knew it was coming off permanently after BIQ, in exchange for much more fashionable (in my opinion) headdress ... the balmoral.
That said, I did exactly what was suggested, to get it looking at least not "new". I soaked in water, and sometimes would have to do it again a few weeks later as it went back to it‘s original crap shape.
Mine was also way too big, and the flap came down way over my ear. Oh well! It‘s hardly functional headdress anyway.
My advice -- join a Highland regiment. A balmoral at least keeps the sun and rain out of your eyes a bit.
