If someone is choking you from behind, tuck your chin in to protect your airway and try to use the stamp on the foot self defence move to make them loosen their grip. 

If the attacker grabs your wrist (especially if they are stronger then you) aim to strike them with your free hand in the eyes or throat. Do not yank your wrist away, instead try twisting your wrist out. 

If cornered in a small space, use an object such as a bag or umbrella use it to create space between you and the attacker.

If you are on the ground the priority is to get up as quick as possible. Use your legs to create distance by kicking the attacker where they are vulnerable e.g. groin or knees to destabilise the attacker.