Most favoriable collision with a car? How should I act before collision, supposing enough time?
I'm going to answer the "if a collision is going to happen" question, not the "how do I prevent collisions", because that's addressed already. What advanced cycling skills are useful for a commuter or city rider? and Techniques for minimising injury when falling or crashing? both have safety tips. Although a more general "safety equipment we should all have" wiki might be handy.

I'm also going to assume they're not actively trying to hit you. Although since about 1/3 of fatalities involve a drunk driver this may not be entirely correct.

Collision from the rear

Despite being one of the most feared collisions, this is also relatively rare (although disproportionately fatal) at about 1.4% of fatalities. If it's going to happen you're almost certainly better off riding off the road to avoid it - crashing into something at bicycle speeds beats being crashed into at motor vehicle speeds.

Collision from the side

If someone is going to drive into you from the side I really don't know what you can do. Lifting your legs above the impact zone or even jumping onto the vehicle might help, and at least reduce the chances of you being crushed underneath it. My focus would be on quickly deciding whether to brake to avoid it, or sprint to avoid it. Then do that. Avoiding the crash is always the better option.

Crashing into something

This usually happens when a motorist turns in front of you or crashes into something in front of you. The cliche "superman" crash, where you fly over the car leaving your bike behind. As with motorcyclists, the best advice is to jump, because if your legs hit the handlebars and break you'll be in a lot of pain for a long time. Better to jump over the car, land on the other side and lose some skin. I've done this a few times and cracking a bone in my shoulder is the worst that's happened.

The advice in the linked questions above is definitely relevant here. Wearing reflective and highly visible, abrasion-resistant clothing will help, as will cycling gloves and decent shoes. A helmet is also handy. I have dynamo lights that don't turn off and a blinking red light on the back of my helmet that I often use in daylight as well.