I've updated this piece of advice to make it more useful. Feedback is appreciated.
1) Do plenty of past papers.
Initially past papers should be done to identify skill gaps to be plugged with revision. Later on past papers should be done to simulate actual test conditions.
2) Hold yourself accountable.
Measure your progress not by hours spent, but by tasks accomplished. Make a list of the things you've done each day as it will spur you to do things in the next day. I kept a track of how many past papers I'd completed.
3) Create peer coaching sessions.
Take the initiative to arrange peer coaching sessions, where you each discuss an approach to a problem you're struggling with and take it in turns to ask meaningful questions to help solve the problem.
4) Revise independently.
You cannot work together during an exam, so it follows that you should spend most of your time preparing independently as well. If you are going to work together, I'd use the peer coaching model for a couple of hours.
5) Invest in good stationery.
You will lose time in an exam if you don't have stationery that you can use easily and effectively. Having comfortable pens, pencils, rulers, rubbers, sharpeners, and calculators will save you time that you cannot afford to lose.
6) Invest in good revision guides.
This makes it easy to access important information you need. The Internet has far too much information, and most people's notes aren't comprehensive enough. I'd go into Waterstones to find guides, and buy them off Amazon.
7) Ask for help.
Your teachers and peers should be the first port of call if you get stuck. If they cannot help you then forums like The Student Room are quite useful. Remember that it is too late to get help when you're in the exam hall.
I hope this has helped.