1. Start the process as early as possible. Seriously. It's never too early to start.
2. Spend time researching the programs you are considering applying to - read their website, as well as potential advisors' websites. Read about the location, the weather, the current funding situation. Ask your professors about each school.
3. Don't apply to "safety schools", there is no such thing. Also don't apply to schools in locations you absolutely don't see yourself living in. Don't make choices that will make you unhappy before you even start.
4. Write an early SOP draft and put it aside for at least a few weeks. You may find while writing the draft that you struggle to define your interests. Spend some time thinking about that; it can be a real soul-searching process and you should not apply before you've gone through it and are confident in your chosen field(s).
5. Think ahead. One of the papers you write for a class this year will likely turn into your writing sample next year; get good feedback and revise accordingly. One or more of the professors you are taking classes with this year will be recommenders next year. Go to office hours, make yourself known to them. Seek feedback from them on your work, maybe even on papers for other classes if they are interested.
6. Use the summer wisely. A small RAship or an independent study could go a long way towards getting you some much needed research experience, maybe also a LOR and/or a writing sample. Not to mention how much it'll help you to better define your interests for your SOP.
7. Find out if it's customary to contact potential advisors ahead of time in your field. If so, do it a few weeks before or a few weeks after the new term starts. Don't wait, this can affect your choice where to apply.
8. Don't stress overmuch about grades. For one, there's little you can do to change the ones you already have. Further, the "intangible" parts of the application are so much more important.
9. Revise, revise, and revise some more. Let professors and friends read your SOP for content and for style. Let someone read your writing sample as well. Go through multiple versions, take your time. These things are hard to write.
10. Be on top of things, part 1. I suggest a chart with the following info for each school: (a) deadline, (b ) app fee, (c ) link to app website, (d) username, password for website, (e) requirements (how many transcripts, GRE/subject GRE score, TOEFL score, LORs, SOP prompt, writing sample length, other - diversity statement, personal statement, letter of intent, etc.), (f) potential advisors, links to websites
11. Be on top of things, part 2. Have a time line: deadlines for each school, when to order transcripts (how many), when to send out application packets, when to contact recommenders, when to send reminders. If you're international, look up American holidays around when you expect to send your app so you're not surprised by the (lack of) operating times of the post office and the schools.
12. Be on top of things, part 3. Get in touch with your recommenders early. Prepare a packet for each of them with your transcript, a paper you wrote for their class, a draft of your SOP, a list of the schools you're applying to with their deadline. Ask them if/when they would like you to send them reminders. Consider having a backup plan for flaky recommenders - in particular ones that will be away and will be hard to track down if they disappear.
