In the US. As I do more irl stuff, I am becoming concerned about being arrested at a protest or whatever. I've not yet come anywhere close to this happening, but should I do some prep like this ahead of time? Are there PSL approved lawyers or something, how should I know who to call? Will I need to have the number memorized? Can I cold call them post arrest or should I form a relationship first? I don't want to get billed.
The National Lawyers Guild should be in your radar: https://www.nlg.org/about/
They provide chapter based defense hotlines. It's never a bad idea to have their 800 number written on your arm in sharpie before an action. https://www.nlg.org/chapters/#massdefense
Replying just to bump this comment
They may also have an "outside of jail" and "inside of jail" number for your area
This is what I came to say. The NLG is what you want here. Write their hotline number on your arm and the number of one contact that isn't at the protest and isn't particularly involved with stuff (so they cops can't try to nab them on something too). This way you have legal help (the NLG) and a contact who can pick you up and/or bail you out or coordinate bail. Make sure you coordinate with the emergency contact number; tell them you're going and when so they know if they get a random call to pick it up. Don't write names, just the number. Write it somewhere you can see without a mirror or something and in as permanent ink as you can, thick and dark (you don't want it to smudge or rub off). This is helpful too in case you're injured enough that you can't call them yourselves, it's helpful that someone else can at least contact someone to help you out.
Also if you've got questions about something you've seen or heard during a protest, find the NLG rep before, during, or after the action. They'll be there as long as they can be, you'll recognize them from their bright green hat and the fact that they're going to be near the edge of the crowd with a clipboard. Sometimes they dip out if things are getting nasty although I've known extremely based NLG reps caught up in kettles and arrested with everyone else so they can continue to do their work from the lockup. Just incredible people, uncritical support to them and I don't give a shit how lib the organization is, anyone who is throwing themselves into danger like that is my comrade through and through.
Source: I have a few friends who do this and I tried to do it too but the process is really hard and in my area at least involves a lottery, I never had the opportunity to try. Someday maybe, the NLG is cool as fuck.