Remember you are not your scores. If your grades are not great, make sure you have something else to offer! If your cut out for it and you really want it, your EC activities should show it. BUT Do have a backup plan. I've known 'better applicants' who haven't got in the 1st time around. Why me over them...?
Different schools are looking for different types of people; students who will click with their current student population and the goals that they would like to achieve.
Apply as early as you can. The application process is almost like another class so make sure you can set some time aside to work on them. Also have friends, family, professors and advisors edit your essays and applications to catch any minor mistakes.
Apply to the schools you could see yourself at. Don't waste your money on a school at you wouldn't be excited to attend.
Pray! My faith in Christ and His will for my life helped me more than any score on a test!
Look on SDN for feedback. If you have an application success story, put it up here so others have hope!