I'm an odd duck, it seems. I put the kingdom cards on the outside. So, it would look like:
{01} {Po} {Cu} {Ag} {Au} {Pt} {10}
{02} {Cr} {Es} {Du} {Pr} {Co} {09}
{03} {04} {05} {06} {07} {08}
I couldn't say why I started doing it this way. I think it was because kingdom cards were generally bought more often than treasure cards, so they were kept more accessible. As the game grew, we just kept it this way.
Like others, I arrange the cards in ascending cost order.
Edit: Oh, and I keep the mat in front of me with my deck. I make sure it's not too close to my discard pile so that if a card gets knocked around, I don't wonder if it was on my Native Village/Island or not. Trash is usually in some dark corner away from the kingdom cards and not too close to any one person's deck (lest someone wonders why he ended up with 20 Coppers, 9 Estates, and 25 Curses).