One thing you can do to avoid making silly mistakes is to become more aware of the types of mistakes you make and catch yourself before you make them.
For instance, if, when you are doing simple addition, you often add things wrong, be aware that you do, and use that awareness. When you go to do simple addition, remind yourself, "OK, I often make mistakes when I do addition. So, I have to be careful here."
Something that I personally found useful was realizing that I would get excited and make mistakes when I figured out how to answer a question. In other words, in answering a question, I would do all the hard work, and then have to make a calculation or two to arrive at the correct answer ,and I would blow the question in those last two calculations. So, now, being aware that I do that, I remind myself to be careful with those final calculations.