I used to have this problem for biology - mainly that I knew the answers to the questions, but it wasn't the answer they want to be given. Practise exam questions and reading examiners reports might help for that kind of thing. I still find it difficult sometimes, but once you get the hang of it its fine. And also, I know lots of people probably won't agree with me on this, but I write down EVERYTHING that I think might be relevant. That avoids any of these 'don't accept unless its preceded by X word' marks.
What are you actually losing marks on on your past papers?