Techniques to remain focused while marking examinations
There is two issues in your question.


How to be fair to each student when it comes to this boring task of evaluating them


My own policy is the "mark-one-question-and-shuffle" : I correct the first question/problem for all the students, then I shuffle the whole stack and go for another question that I also pick at random. This way if I am tired or in a bad mood, it will impact everybody, so it will be fair.


How to stay focus for a long marking session


I cannot answer to that and I guess it depends of many personal factors. Personally, I find marking tasks boring but easy to do and easy to focus on. It's like driving, some people can drive for hours, others can't. To tell the truth, marking is somehow relaxing for me.