I'm an active German antifascist. Here's something I do:

0) Get a lawyer. If you're arrested and you don't know a lawyer, you're screwed. And learn your rights: what do you have to tell the cops, and what you can refuse to tell them. Always carry a valid ID card with you.

1) When publishing pictures, especially on Twitter: place stickers over people's faces, or better: pixelate using ObscuraCam. The best thing is of course to not take pictures or video at all.

2) Get a "burner dumbphone", best are used, old Nokias and a burner sim-card when going to demonstrations. Do not activate or use the phone at your home or at meeting points.

3) If you insist on carrying a smartphone, get a recent Android phone with support for FDE and an exchangeable battery. Enable FDE, also on your SD card, and in case you're about to get arrested, take out the battery or drop the phone to the ground so that the battery falls out and the cops cannot use imaging devices. Use a strong passphrase. iOS devices may be secure, too, but they have the disadvantage that you can't pull out their battery or switch them off in a hurry. If you care about your device, get an IP68-proof/rugged device - cops don't care if they damage your property when pushing you around, and it's easy to e.g. fall on your phone when you're pushed to the ground. Android: disable USB debugging, or if possible with your model, the entire USB stack. On a rooted Android phone, you can do so via an adb shell command.

4) When browsing around the web researching political stuff, use TOR. Do not download unneccessary stuff onto your computer.

5) Securely encrypt your computers and all external media devices (USB sticks). OS X can use Filevault, Windows can use Bitlocker. USB sticks are best protected by VeraCrypt (as it is a cross-platform solution). If you have a NAS that doesn't support encryption, ditch it and buy one that does.

5) If you receive sensitive information, delete it as soon as in any way possible. Insist on communicating via GPG-secured emails, and password-protect your key. Written information should be shredded to as tiny pieces as possible - don't burn the paper, ash flakes or incompletely burned paper can be restored (as evidenced after 9/11).

6) Enable 2FA, preferrably via a token generator app on your phone, on any service that supports it. Store the backup keys (you will need them e.g. if your phone gets damaged!) somewhere safe that is NOT your home (e.g. at your parents' house). Do not label the sheets with a cleartext name of the service/account associated with them. SMS 2FA is the "last measure" as you'll be vulnerable to government attacks, but better SMS 2FA than simple password protection.

7) Handle sensitive information on a strict need-to-know basis. And for heaven's sake, don't talk about planned actions in public. Or brag about things you/your friends did or plan to do - while bars etc. usually aren't crowded with agents, someone may decide to rat you out to the cops.

8) Before going to any demonstration, write down the name and phone number of your attorney with waterproof ink on your arm. That way you don't have to rely on the cops finding your attorney or delaying calling him by taking their sweet time to do the search.

9) Inform close relatives/roommates that you're away, especially if you have pets, children etc. that need to be taken care of. Have enough cash on your bank account (or have a relative) to pay rent if you end up arrested.

10) don't ditch fares, or if you have a car, always take care that it's up to code, legally registered, and taxes/insurance are paid. Nothing sucks more than getting arrested for petty stuff, and pulling people over for broken lights is a common excuse of cops to search the vehicle. Do not carry huge amounts of cash in your vehicle (google for "asset forfeiture", it's really gross what cops can legally do).

11) don't ever go drunk, intoxicated or not well-rested to any political event. Do not take drugs of any kind with you, except medicine that you need (and for these, best take the original prescription or a copy with you, so the cops can't bother you with drug charges). Preferrably use plastic glasses (glass lenses can cause grave eye injury when damaged), contact lenses and cosmetics of any kind tend to aggregate nasty stuff like pepper spray.

12) Always take sufficient supplies of water, food and a small pack of glucose tablets (in Germany, we know them as Dextro Energy) with you. If you can, take a couple small adhesive bandages with you, and go to a First Responder education (this is useful anyway, even if you're not "actionist" - you can save lives!)

13) Connect with other political groups both in your area and state/nationwide: ACLU, antifa groups, civil rights movements. Political parties (liberals, greens) may also be of interest to you, depending on your focus.

14) Beware of snitches or agents provocateurs that try to incite you to violence. When you want to go the "actionist" route, be aware of the potential consequences if you get caught and don't do anything you're not comfortable with.

15) Do NOT go on political demonstrations with firearms, knives or other weaponry. In most jurisdictions it's illegal, and even if it's legal to assemble with arms, it's not sane to do so. When you see armed protestors, or a demonstration turns violent, GTFO as fast as you can.