The basic reasons of silly mistakes are overconfidence and lack of practice. So, following things may help:
1. Put your pen to the paper. Yes. Try not to solve questions mentally and solve them on paper. This often eliminated the silly calculation mistakes.
2. Read the Question Carefully. Understand that almost Every single word in Quant is important. So, pay special attention to statements.
For instance: a, b, & c are integers

meaning that they can also be Negative..
3. Practice more questions. There is no denying that Practice makes us perfect and in addition to perfection, it is one of the best ways to eliminate silly mistakes.
4. Re-check your answer. Though it will take an additional 5 to 10 secs, it is better to spend that additional time than to get those easy/medium questions questions wrong..