If I'm not at home, I try to hold it in until I get home.
At home --- lots of angry, sad crying, and talking, talking, talking, pacing, pacing, pacing. I pace around and around, talking to the four walls, "thinking out loud" basically.
If it's a person who upset me, and I didn't think on my feet fast enough to defend myself, explain myself, right a wrong, or whatever else upsetting that happened in the encounter, I start saying out loud what I wish I'd been able to say on the spot to that person. But I tend to go over and over and over the same points to myself.
Same with a general situation I'm breaking down over -- I pretty much "write a diary entry" but verbally out loud to the thin air, talking about my rotten day, week, month, year, life. LOTSA talking out loud to nothing and nobody, accompanied with a lot of angst and tears and repeated points.
When I can finally get myself to calm down and stop the pacing and the self-talking, I usually try to sit down and watch something -- I love movies and my favorite TV shows, and usually have hours and hours of recorded stuff on my DVR, so I'll pick out a movie from there and try to get into it. If I pick the right thing I will usually be able to be distracted enough by it to calm down. However, I will still do things like pick at the skin on my thumb (a lifelong stim, my thumb is now hugely calloused and ragged), pick at my scalp (bigtime) or twirl strands of my hair obsessively, even while I'm enjoying the movie. But I tend to do those things even when I'm not consciously feeling particularly upset, too. I just do them even more when in meltdown.
I also kiss my cat, who is an affectionate one. Or look at, play with, admire, preen and cuddle some of my toy stuffed animals. You're never too old to tickle the ear of a toy puppy.