I think it helps a lot if you know you plan on submitting at some time in the future to start preparing now in small ways. Some things I did:
*Whenever you get a request to review for a journal, graciously accept. But before you hit the "I will do this review" button, contact the editorial office and ask if they wouldn't mind sending you a personal request letter that includes your full name, and a statement like "Because of your expertise in the area of XYZ, we are requesting your services as a reviewer". They are more than happy to do it, and it's just inserting one sentence into boilerplate text. Some people have tried to submit requests that were passed on to them by their supervisors, or that say "dear colleague" or "dear researcher"-- these won't work.
*Start Googling yourself regularly with every version of your name. Not just Google scholar, but regular Google too. I found a few news items in small trade-type publications that I printed out and kept in a file, and then half a year later I could no longer find them on the web (some smaller news publications don't necessarily make news archives available for a long time). Do this every month or so, and you won't miss anything that may mention your work.
*When you are at scientific meetings, don't be shy about asking people who approach you about your work (ie specifically come to your poster or sessions with questions and genuine knowledge of the topic, not the "just passing through" people) if, at a future time, they would be willing to write a brief letter indicating that they sought you out at a scientific meeting because of your work. These can supplement your reference letters;they don't take their place but they are certainly helpful to document your fabulous reputation!
* Look for opportunities to serve as a grant reviewer for institutions/agencies other than your own. Some agencies (e.g. Susan Komen foundation for breast cancer research) have an online process where you can nominate grant reviewers. Have someone nominate you, or in some cases you can nominate yourself.
*The thing that saved me the most time when I was putting my packet together: Print and save EVERYTHING that might possibly be part of your packet now, and keep it in a safe place. Note the date and url (if applicable) of where you got it. Keep subfiles of which category things might be considered for. Start now, and even if you don't apply for a year or two while you get your 2 yr residency stuff sorted out, you will have much less work to do when the time comes.
Good luck!
