What a great question - I am always looking for new, more effective ways to manage emotions/ride out the tough times; I'll look forward to going through the replies, seeing the new ones as they come in.

I like things I can access anywhere - work, school, etc - so many are simple.

Using DBT has been a literal life saver for me. I was so lucky to have 2 go-rounds of DBT training through county mental health, and it was a god send!
I built a virtual skills card, and also skill flash cards, that I can review/complete/refer to when I need some help.

Keeping attuned to my body and moods has also been pretty essential. I use a Mindfulness Bell app, set it to ring every hour. It's a nice, temple bell tone that is not annoying, set it on low so I can hear it. Every time it goes off, I scan my body for any tension I'm holding, ask myself what i am thinking/feeling right now, and do a "sensory scan": what do i hear/smell/feel/see?

Visualization: before I get out of bed in the morning, I think of some goals for the day, visualize myself dealing successfully with situations I find challenging (social situations, coworkers, etc). I try to rehearse, go over what has not been helpful, try to think of other ways to deal with things.

Spreadsheets; besides the DBT card I cobbled in Excel, I also have a Self Care tab I try to complete every am/pm. It just takes a few minutes, but allows me to keep track of things like showering and other basic hygiene that can fall by the wayside when I get too depressed. It's a sad state of affairs, but I find it helps me care for myself when I don't really have the will to do so, if only to check it off the list and get it over with.

Talismans: I carry some small things that tie me to the people I love, and make me feel more secure when I hit a low point; for example, a silver dollar sized purple quartz heart my dad gave me that I slip into my pocket every day. He had sent it with a supportive note, and when I feel small, just touching the heart makes me feel comforted, like I matter to someone.

Visualizations, pt II: this sounds kind of nuts, but I have crafted a serene place that I can go to in my mind when I need to calm down, soothe self. I can go there even when I am at work, and get stressed. Its not total dissociation, I just use it for a few minutes when the stress gets too much, emotions are on fire.

I have a pretty fatalistic, pessimistic world view that I try to keep to myself, and try to keep myself from lingering on. So much of my self care involves me putting off emotion based urges/actions for at least a day to prevent damage/overreactions, and ensuring I remain calm and rational.