Funny, our coping mechanisms differ a lot.
Though I do regress into my shell, I also don't tend to go too deep - isolation really messes up my mental health and the last thing I need to hear when my mindset is doom-and-gloom is my own thoughts.
What does help is picking specific friends who I know can pick me up in my bad moods. I avoid friends who I know can mess up when I'm not in a stable state.

Also, generally I'm a very messy guy and a relatively lazy cook.
So I tell friends that if suddenly I'm cleaning a lot and doing chores, I'm probably not doing too well. It's a little attempt at "I can't control X but I can control this".