First, aim higher. Aim at more than 700. If you aim at 680, it is likely you won't get 680 in the real exam.
Then have a plan on the study materials, the order of studying them, and a study routine! Each and every day, you need to put in 1 to 2 hours. If ur current schedule does not allow this, then alter it, e.g. get up early. Be serious about the exam in your life.
Have a record on what you have studied, errors, doubts. And review them once in a while during the course of studying.
Find your weakness. Is it math or verbal? in which specific area? train it extensively, but don't ignore the rest.
Find your strength. This is your insurance. In a bad day, this insurance will always deliver. And it is ur insurance that ur score will always be at a certain level. e.g. my math is good, so even if I did badly in verbal, my score will always be above 650.
Finally, participate in this forums. Keep in mind that what you do in this forum may be repeated in your CATs, so your CAT score might be scaled higher. but u will see a lot of good questions u cannot find by yourself.