For RS it depends on your course largely. Are you doing OCR? We did OCR Philosophy and Applied Ethics. As long as you know the basics and watch a lot of news/know your case studies/just know stuff which is relevant can get you the grades.
For my GCSE RS there was a question on animal testing- you HAVE to make it a two sided essay. If you just say 'it's wrong' and back it up throughout you won't get all the marks. It doesn't have to be even though. Make sure you weave your own opinion in but also consider teachings of the religion you are studying etc. For my argument for I mentioned thalidomide testing and how a lack of complete animal testing caused a lot of grievance in pregnant woman and babies being born eith deformities. Etc.
Dunno about other boards. And also if your doing OCR make sure you spend like no time on the first few qs. Q 1&2 can be bullet point answers. Q3, if they ask for 3 things it can be bullet pointed, if not, keep it brief. For 6 markers you need detail, 12 markers approx a side, a bit more wouldn't hurt but no less.
Maths- practice loads, and get a good scientific calc, not the rubbish ones schools reccommend because they are cheap. Mine was Casio fx-991esplus and it cost more than everyone else's calculator (£18) but you can differentiate on there and solve quadratics which were big parts of my GCSE maths exam. I recall a 6 marker which involved quadratics and that calculator made life SO much easier. But yeah, practice loads, do harder qs from textbooks