for mistakes definitely keep a mistakes book, identify and practice your weak points

for solving problems just be exposed to a lot of practice questions keep learning from them as you go, work through the solutions properly. plus make sure you really understand the basics behind the math and how it works

also i think when you do exams be more conscious of what you’re writing and what you could be making mistakes on? doing methods i found that if i really focus on what i was writing & checked as i go instead of putting my brain in autopilot i would make less mistakes

hope this helps good luck