For the sciences, use as many past papers as you can get your hand on, doing the most recent/relevant ones first and working back until they are from about 6-8 years ago. Learn the answers that keep cropping up, you'll spot them as you start doing them.
Make sure your algebra and understanding of equations is good, as this can score you a lot of marks in both maths and physics.
And use the CGP bibles, they are truly amazing (make sure it's for your exam board/specification though!), although I wouldn't recommend them alone for A Level.
For French, I'd recommend learning vocabulary lists, maybe 10-20 words a week, that are from your specification (eg; school items/house items/adjectives etc), as even if you don't get the tenses right, you can get a lot of marks just being able to translate vocabulary, and 9 times out of 10, you can guess the tense in the context of the question (Jaque is going to the shop, what is going to buy? Will be future tense).
And ultimately, try to enjoy the subjects you want to do well in