Until these commands are in my muscle memory, I keep them here.
C-b means, of course, the prefix key.
C-b %split pane vertically (into a left and right pane)
C-b "split pane horizontaly (into an upper and a lower pane)
C-b <arrowkey>switch to another pane
C-b C-<arrowkey>enlarge the current pane
C-b qshow pane numbers (press key to jump to pane)