Search this forum for NP questions and see how people used logic (as opposed to just plugging in numbers) to solve these problems.

Do the same thing with OG, pay attention to how logic is used to solve these problems.

If all else fails, then plug in to clarify concepts for yourself.

I used to ONLY plug in, and it took a long time. Now, I use a mix of the two, plugging in and logic.

AleksandrM gives some good points, but if you're hungry for more problems and down-right fundamental knowledge on Number Properties, you could try the Manhattan GMAT series specifically covering NP.

Note: I usually don't plug products! But, MGMAT, if you're listening, I could use a few dollars to help cover tuition!

I found it to be helpful in some areas, the others were already well-learned thanks to de rigeur training from math classes during 17 years of study.

I would also stress the importance of categorizing a problem as soon as you can: prime numbers? exponents? consecutive integers? etc.

The sooner you can recognize the problem type, the faster you'll be able to implement. It also fosters pattern recognition, allowing you to draw connections to similar problems you've seen/done before.

beeparoo, thanks for the plug! (Wish I could do something about that tuition... thankfully, tuition pays for itself in the end, especially for b-school, right!)