I had my Modern Studies prelim yesterday and found it was OK. Timing is crucial in both papers, make sure to only do the easy 10 mark questions for 15 mins then move on to the report.
The headings we are told to use:
Introduction
Arguments for my recommendation (6 points, at least 2 BK)
Arguments against my recommendation (4 points, 1 BK)
Rebuttal (optional section if you have rebuttal in with the arguments against)
Conclusion (bullet points)
Some people have Financial Implications after Rebuttal but if that is also included prior to that then it doesn't need a separate section.
For memorising essays what I did was gather all the essays we had done and then I wrote them down on a separate piece of paper each with quite basic descriptions, so that it was easy to manipulate the points so I could put them into a slightly different essay.
I wrote points out on stickies and stuck them around my room (I was going to do it with statistics too but I forgot). And I wrote out the essays in note form on index cards and memorised them like that.
Practising them over and over helps keep the statistics in your head, too.
I'm possibly too late to help you now... good luck and hope it went well