The GRE is a test of your math and verbal skills, AND it’s a test of your test-taking abilities, which include endurance and time management. So, be sure to take several full-length practice tests as part of your preparation. It's best to achieve your target scores on at least two practice tests before taking an official test.
Note: If you purchase The Official Guide to the GRE Revised General Test (a must-have resource for all GRE prepsters), you will get two full-length paper tests as well as a CD-ROM containing the official PowerPrep II software, which contains another two practice tests.
Below are more sources of GRE practice tests.