Check your surroundings, make sure you cannot be cornered and snatched out.
Invest in a good telescopic baton.
Flank in order to throw projecticles properly, avoid just smashing their shields.
If the thing is heavy or the throw nicely arced, you might land a shot over their shield, but they do have helmets so it's likely to take in the impact. Doesn't work in a testudo (shield turtle, shields over shields, etc)
Shields ain't for pussies, snatch one if possible or make one.
Don't roleplay a hero, bail out when things go south, getting snatched is one of the worst case scenarios. Based guerilla, retreat to fight another day as they say.
Covering your face is one thing, get yourself some goggles, gas mask, anything to hamper effectiveness of tear gas and smoke grenades.
Avoid clumped up formations unless participating in a direct assault/push or simply moving forward in a line.
Protective gear as mentioned is nice, but stay quick, agile and mobile, manoeuvrability is your biggest advantage.
Keep check of your stamina and work on it in spare time, bad positioning whilist out of breath leaves you vulnerable.
Bandages and peroxide are your friend and a friend of your friends, look out for fellows in need.
But first and foremost don't get hit with a slug shot. Shotguns, although giving the impression of spread fire, can be surprisingly accurate with the right gear and hand.
Having spare clothing in your backpack comes in useful when you need to bail out of your black bloc appearance to blend back in. Also good for makeshift bandages and slings for injuries.
All I can think of for now, sorry that it's not completly defensive, but it's better to know your way around offense if emotions and adrenaline get the better of you.