Don't be too fast while solving the problems. Even if you are strong in Quant section, go with a balanced approach. Be careful with the traps. More balance is required especially in dealing with DS problems as we are more prone to traps there.
After completing all basic material,try out some advanced Quant books like Manhattan etc., to get more varieties of flavour. Go through the challenge Q's, 700-800 level Q's on prominent sites and materials.
All the best

If I were you, I would create an error log and identify each type of error and try to understand why I solved it incorrectly even though it was a simple problem.

That way, I may be able to find a pattern. From my personal experience, I used to mess up a lot of DS because I used to misread the questions - E.g. Is a>b? type question. Initially, I used to mark "Question cannot be answered using both choices", in cases when I would conclude that a was less than b. That meant that I knew the calculation but was making a silly mistake of not understanding that the answer to "Is a>b?" could be "No" as well. You may be doing really well in such cases, but I was trying to explain that when I identified through an error log that I got a ton of such questions wrong, I worked on it and fixed this personal problem.

Hope it helps!

Apart from what others have suggested, I think that many of the silly mistakes committed in the QA section of GMAT are because of not reading the language of the problem carefully.

The Importance of Reading the language of a GMAT problem:

(1) Your strategy of approaching the problem will depend on how well you understand the problem.

(2) Small words such as 'atleast', 'positive' and 'distinct' are very important. If you miss out on reading them, you will fall into one of the traps laid by the person who designed that question.

(3) Read what is asked to be calculated very carefully. If the question is asking you to find the ratio of the radius of the larger circle to the radius of the smaller circle, it is possible that you've calculated the ratio of the radius of the smaller circle to the radius of the larger circle first. And you might get so pleased with your effort that you will go ahead clicking an answer that will definitely be among the options.