My man, I was in the same boat as you. I used to get 8 out of 10 NP questions wrong. At this point, I am getting about 7 out of 10 right, and that is after a lot of work. Here is what I did.
I got the NP guide from Manhattan GMAT. This resource will START you off.
Next, go over the OG orange guide and just read and re-read all of the numbers you feel include number properties (do not rely on their assessment of what constitutes number properties because some of the NP problems will be marked something like First order equations + inequalities).
Finally, do not forget to use the plug-in method. It SOMETIMES works. But, be careful. Always suspect NP problems...ALWAYS!
