Based on this forum post: https://groups.google.com/forum/#!topic/ggplot2/mK9DR3dKIBU
Sounds like the easiest thing to do is to add a line break (\n) before your x axis, and after your y axis labels. Seems a lot easier (although dumber) than the solutions posted above.
ggplot(mpg, aes(cty, hwy)) +
geom_point() +
xlab("\nYour_x_Label") + ylab("Your_y_Label\n")
Hope that helps!