There is a book specially written for such struggles, titled Pass Your Weak Subjects, by Richard Palmer, 2008.
BiologyUse mind maps, diagrams, flow diagrams. They contain a lot of information and help in visual learning.
ChemistryMaster the vocabulary.
Use mnemonics, devices that help you retain the new information you've just picked up.
EconomicsUse mind maps
Use revision notes e.g. Cliff, Barron
Practice active recall.
MathRevisit the topics constantly.
Do lots of examples. Study worked examples.
Stop when your targeted load/quota is done, celebrate. Don't burn yourself out in ambition and greed!
Use colorful imagery.
PhysicsPay special attention to and master key equations and phrases. Such phrases cover key concepts.
Master key math concepts and skills.
Use revision guides. e.g. Cliff, Barron, Schume