ggplot: How to increase spacing between faceted plots?

Use the theme function:

library(grid)

p + theme(panel.spacing = unit(2, "lines"))

See also here: Slicing plots generated by ggplot2

Just to add to @rcs response:

# Change spacing between facets on both axis
p + theme(panel.spacing = unit(2, "lines"))

# Change horizontal spacing between facets
p + theme(panel.spacing.x = unit(2, "lines"))

# Change vertical spacing between facets
p + theme(panel.spacing.y = unit(2, "lines"))