Head to the best possible spot available even though it may seem crowded from a distance - for Yokohama, the delicious spot was near the water. The ground is covered with blue or green sheets with kanji written on them. These are spaces which have been "reserved" by others - it means "I-got-here-first-so-don't-even-think-about-it." Even if there is nobody sitting around the sheets, most folks respect that the area has been taken by others - just like how folks over here leave their bags unattended at Mc Dees.
You need to take off your shoes and just walk on the edges of other peoples sheets to get here. Once here, you will see that other folks have not placed their sheets evenly in rows - meaning that there are gaps all over the place. The very cute girl you see in the bottom left of the photo was sitting with her friend in one of the empty unclaimed gaps.