Do lots of textbook questions and every time you make a tiny mistake (even if its forgetting to put a minus), write it down.
You should have a list now. Every time you do a question next time, check with that list and see if you think you could have repeated any mistakes. Once you're sure that you have stopped making a particular type of mistake, you can cross it off the list. Keep doing this until you have nothing else on your list.
Oh and also when doing textbook questions, don't do the question, then check answer, then do another, then check again etc. Do a WHOLE exercise without checking solutions. If you got even the tiniest bit wrong and you did the whole exercise incorrectly as a result, do the entire exercise again. This 'self punishment' system forces you to remember it for next time. It has most surely worked for me. I must have done the Cambridge series at least twice because of this, and it cut my silly mistakes by more than 90%. I now only make silly mistakes if I'm really tired, or if I'm distracted by something else.