Aglages is right. The biggest hurdle over the summer is getting your recommendation letters. Get those now while you still have access to your teachers. Also very helpful to get their contact information (cell/home numbers and email address they check regularly). It was not uncommon for both my DS's to have to make edits (informational, grammatical, etc.) and have the teachers correct them. The more letters you get, the better. You can then choose which suit the circumstances better. (You'll be sending a LOT of these letters out). In some cases, maybe only three are allowed. In other cases, you may be able to send more.
I might also advise you to provide teachers with a list of topics to address. We all know it's leadership, academics, attitude, civic involvement etc. but your teachers may not. Also gives them a "checklist" so they don't inadvertently leave something out.
Start working on your essay. This will be a "Why I am Seeking an Appointment" document. Work on it, fine tune it... THINK about it. This is also a good soul-searching exercise. This essay, or some edited form of it, will be used several times during the application process.
Document all your extra-curricular activities during your high school years and keep adding to it during your senior year.
The rest of the summer process will involve the MOC process and completing portal requests as the come.
As in all other things, the earlier you complete each step the better.
Good Luck!