People normally write down/make a note of the problem and the mistake they committed while solving. If you go through these notes like you do for flashcards, then you are likely to NOT commit that mistake ever again in exam. This is one way of reducing silly mistakes.

Most careless mistakes come from two places - either the perceived need to hurry up and get to the next problem or becuase you believe the math is well within your abilities so you don't write anything down. Analyze where your mistakes are coming from.

There is a big difference between rushing and moving quickly and methodically - rushing never helps on the GMAT - if you hear that little voice yelling at you to hurry up then you need to begin to ignore it - tell yourself that you have all the time you need - you've been practicing for a long time. Imagine if I calmly asked you to go pack your bags for a weekend trip - you would go evaluate what you need - probably walking from your bedroom to your bathroom - thinking through things step by step it woudl take you a small amount of time and you'd have a great weekend. Now if I ran in and said "quick, hurry, we need to pack and be out of the house in 15 minutes" - you would probably react very differently - running around grabbing things that come to mind first and at the end you would have forgotten your toothbrush . That is what you are doing if you keep telling yourself to hurry, rush, get to the next question and it doesn't help.

The other type of error is when you so steps in your head because they are "so easy" - the easy fix to this is to write every step down - it really doesn't take any longer and you will catch yourself making silly calculation errors.

Hope that helps a little bit.