Having a good longterm therapist is the number one for me. I see a private one, and have a good relationship with her, and I could contact her in a crisis outside our sessions, too, if I needed to.
I'm back on medication again after trying once again to quit it, and things getting worse. I sought out a GP who I heard is decent with mental health issues, and I see her fairly regularly, since I don't qualify for CMHT care.
In a bad crisis, it's hospital for me. Luckily not been an inpatient on a psychiatric ward for more than ten years. But I do end up seeing the RAID team in the hospital every now and again, and that intervention (and meds they've given me short term) have tended to bring the situation back to a more normal level. I now have a good PRN medication in place that's prevented me from getting that bad.
As to preventing things from getting that bad...
I have to be careful with my stress (too much) and sleep (too little) levels, as those are the two things that will affect my mental health really badly and can easily lead into a crisis. This means that I've had to arrange my life to minimize stress as much as I can, and I use sleep hygiene and meds to make sure I get enough sleep.
I talk to DH about my mental health, and it helps to be open (with the right people!) about it. I have other supportive people in my life now, too, who I don't need to pretend for. I also frequent a peer support website.
Self care for me also means trying to eat mostly healthy food and regularly (I have an eating disorder), to get some fresh air regularly, and to do things that bring me enjoyment (crafting, audio books, podcasts, games, an art group I attend, catch ups with friends whenever possible, a box set with DH, a long bath with a magazine etc).