a) If you're adventurous, try approaching from the side, or even going around the back, you can really get some great shots.

b) Tie some rags around your camera to absorb some random blows and also reduce the attention it attracts. Gaffer tape can also help to make it less attention grabbing.

c) If you're using an SLR, lens hoods and a clear filter are a good precaution to protect the lens from flying objects.

d) Onions can help with tear gas a bit, obviously cloth and masks can be good too, but they may attract unwanted attention.

e) Find a position near a wall, to protect one side of you, stay low and if possible drag along a friend to watch your back. (Back to back is often good for photos for both.)

f) Be prepared to have to wait out the disturbance, more photogs get injured trying to leave in the middle because of deadline than whilst actually shooting pictures.

g) Your reaction to the police will be largely determined by their attitude. I know you Alex are going along as an independent, so it's most likely they will be hostile. Try to stay a little bit away from the front line, ready to run even, because if they catch you they will probably want to smash the camera. (Ironically, film was in good thing in this case, shoot a few, change rolls and hide the roll on your person.) If your camera is small enough and they catch up with you, try to hide it on you. Into the underpants is a good bet if you wear loose trousers. Once, doing so protected my vulnerables from a swift kick, which was a lucky trick.