Have a plan!

Check who is on your pk list every so often, and have a plan for what you'll do if they attack/approach you.

It might be:
- run a mile,
- use skill X,
- flee and teleport.

That way, if they do surprise you, you won't waste valuable time trying to make a decision. For a veteran, acting decisively without advance warning is easy. For a newer player, it isn't, especially with the adrenaline flowing.

If you already know what you are going to do, you'll be far more likely to do it before it becomes too late.