This is a huge stressor for college aged students, especially as an upperclassman. I'm here to provide some advice to those underclassman looking to apply to Graduate school sometime in the near future. The summer going into your senior year of Undergrad is probably the most vital time.

1. Take a GRE (or whatever standardized test you need to take) diagnostic test to see where you stand. Make sure to sign up for an online practice course, or buy a book to keep yourself motivated. I would suggest taking the test the month classes start, so that you have ample time to retake it if need be.
2. Keep yourself involved with your professors over the summer. Working in a lab, going to get coffee, and helping teachers with their work gets you more experience in the field (as well as putting you on the right track towards a letter of recommendation).
3. Make a list of the schools you are considering and keep researching them to make sure you are interested in the programs they offer. Try to keep it to around 8 schools, including 2-3 safety schools, 3 target schools, and 2 reach schools.
4. Email the professors at these programs you are interested in and ask them about their research and what they are looking for in a prospective student.
5. Many of your programs will have different due dates, applications, and required materials. I would suggest making a spreadsheet to keep yourself organized.
6. Start working on your personal statement for Grad school applications. Most applications will have at least one question pertaining to your personal goals, and you will need to have those well rehearsed and prepared.
7. Create a list of program-specific questions and answers for interviews to practice. The earlier you begin thinking about possible questions, the easier it is to plan out a response.

The most cliché (and important!) piece of advice I can give you is to be yourself. Your enthusiasm and positivity about your interests will shine through to professors.