Focus no further than the Departure section...get it done...
then the next step is the En-route section....get it done...
Joining/Initial Approach...Done..
Precision/Non-precision...done..
EFATO (Engine Failure After Take-Off)...done......
etc.....bit by bit..
This is excellent advice. Also I'd add, once you are in one of the above sections, start to think what's next? What I am using? Radio/nav aids? What will I need next? Plan ahead - stay ahead. LISTEN to the radio. If you can hear London passing off a/c to a frequency that is in the same direction you are also going, dial it up so it's already there in the box when it's your turn - one less thing to worry about when your writing it down.
Be ready for the unexpected. On my IR renewal, I had in my mind what I was going to do on each section of my route however ATC gave me a "direct to" and I missed out 2/3 of my planned route so my plan in my mind went out of the window. But I simply revised my plan and carried on.
I've said it before and I'll say it again, talk to yourself. Some people say some examiners don't like it - tough ****. It's your test, you're paying and if it helps you - do it. Say out loud what you are thinking - you don't have to be giving a constant clear running commentary, just mutterings almost under your breath as you are thinking things will suffice. The examiner can't read your mind and if you have made a mistake, and you say out loud that you have recognised it and how you are going to correct it, that can make all the difference. If you do make a mistake though, don't dwell on it, deal with it and move on to the next bit. That could be the difference between a partial and a fail and a partial is much cheaper than a fail !!.
Good luck.