1. I would have put money aside for the process. As much as I love deciding between paying my roommate back for something and paying for an application fee, I'd prefer to never have to make such a choice again *end sarcasm*.
2. Back everything up. On the computer, on a flash drive, have a damn physical copy on hand. My computer died after fall semester ended and I ended up working off of older copies of my SOP to jog my memory and write my final version for two schools. It must have been good enough, since that copy got me into the school I'm attending, but it definitely tested my nerves.
3. Don't let the process get the best of you. I was so stressed dealing with one school that I broke down and couldn't function. I ended up on my couch one night in the fetal position instead of studying for a physical oceanography test and failed miserably (luckily the class was on a point system and I ended up getting an A after working hard the rest of the semester). In retrospect, the situation was not worth all that. This piece of advice also applies to all facets of life, and I'm slowly teaching myself that before I give myself a heart attack.
4. Stick to your guns. I knew that a masters was the best route for me at this time in my life, but I started to doubt myself as schools started offering me money to switch to a PhD candidate. In the end, sticking to my gut led me to realize that one school in particular was not a good match for me (actually, the same school that sent me into the fetal position), and now I'm joining a program that really cares about me, shares my beliefs and values, and is in a city that I love to death.
I think those are my big lessons. All in all, be prepared, calm down, and trust yourself and your abilities.