I'll echo what Kasia said, and add that any official materials are great for practice (OG12, OG13, OG Quantititative, OG Verbal, GMATPrep tests). However, it's also important to add that, while these are great resources for practice questions, they're not really meant as instructional materials. For example, the OG13 often has solutions that are highly algebraic, when there are often faster approached you could have taken.

To increase your score, you'll need to learn some GMAT-specific strategies (and the application of all key concepts), and you'll find the kind of instruction in a variety of resources, beginning with the various test-prep companies featured on this website.

Cheers,
Brent