The lyrics here are butchered (in "rockerbabe99's" defense, the track isn't that clear so it makes it difficult to decipher lyrics). The correct lyrics are below:
****
Picking up sound on the interstate,
I am my breath, letting in waves.
There will be time when the sleep
I’m in covers me whole, covers me thin.
I know I’ll wake up old,
forgetting which box this is in.
How I will keep you just how I left you.
Our daughter once told me I know a lot.
Now I’m strutting off with more than I’ve got.
A hitchhiker told me I don’t talk a lot.
It made me feel fine, made me quiet.
If you’d of said it right
instead of painting words white…
How I will keep you just how I left you.
Come on, say it right.
*****

As for interpretation, I am at a loss as of now. It is almost as if he is singing about aging, and wanting to keep his memories intact as he gets old. I don't know. Great song though. I hate the Shins, and while this song reminds of something The Shins might play, Blind Pilot does a great job at making a sweet-sounding melancholy song.

appreciate the finer memories in life, don't grow up too fast....?

This song is a story about a man leaving his family. How it all shook out and how it all's gonna shake out.
The narrator, played by Nebeker, hates what he's becoming but does it anyway.

He's driving away, picks up a hitch hiker. He & the now former Mrs. Narrator have a daughter. The daughter thinks he's tops, but he knows better.

I think it's more than that though...
I agree that it's about leaving someone, but I think that it kind of centers on how when you've been away for something for a while, or grown old and let memory get to things, you forget all the details about something, and you just remember them for one impression, one singular idea. You don't realize how they have changed, or all of the things that once tied you to them.

But if you don't 'paint the words white', if you don't blanche out all of their color, their vigor, their passion- if you don't let the memory fade with age, and you remember all of the details, then they will always be to you what they were when you left them.

The same with the hitchhiker and the little girl: He becomes to them what they think of him.

Not really an interpritation, but I just read this here:

http://laist.com/2009/08/03/meet_israel_nebeker_-_lead_singer_o.php

"Was "Go On Say It" based on an actual incident?

Well, it's really a bit of both. The inspiration came from me being really furstrated with a relationship that I was in. I felt like we were parting ways and neither of us really wanted to, you know? She said something to me that out of context was such a reasonable, sane thing, but the way she said it really upset me. I remember thinking, "My whole life after this will be different." Just because of the way she said that one thing. The story in the song is fiction, but the feeling within it is based on that moment."