One thing I've had to hammer in to my year 9 DS this year is to write and say the bleeding obvious in all his answers in all subjects, including writing out equations and every single step of a calculation, even if it was "obvious" to him!
He was starting to get annoyed at, what he thought, was unjust down-marking of progress tests, not just in Maths, but in other subjects too. In his mind, he knows it and was getting it right, and it was really demotivating him. Unfortunately, the school have a policy of not allowing the test papers out of the classroom, so I was working blind as to the problem. After a particularly poor (for him) Physics test, I contacted the teacher and insisted on sight of the test paper so I could see what was going wrong. As soon as I saw it, it was bleeding obvious - that's exactly what he was missing - the simple stuff! He was reading too much into the questions and answering in too much detail missing the basics and with equations/calculations, most were so simple, he could do the calculations almost by instinct, i.e. 50 metres in 10 seconds, what velocity?
So, problem identified within a few seconds, and it's something I've been really working on when I've been helping him with revision or homework and latter tests are showing far higher scores!
In English, for example, he'd be set a homework to compare and contrast two poems, and he'd spend a massive amount of time and get really stressed trying to analyse and interpret them, and whilst he'd get a good mark, it was always only 80% or so, he just couldn't break into the really high marks. So, after seeing what was going wrong in Maths & Physics in particular, I got him to start saying the bleeding obvious in English too, i.e. simple things like that poem A rhymed and poem B didn't, and, ker-ching, 90%+ marks now!!!
Couldn't believe that none of his teachers have highlighted the real problem and just let him carry on making the same fundamental mistake year after year in virtually all subjects!