In the latest version of ggplot2, this can be more easy.
p <- ggplot(mtcars, aes(wt, mpg))
p + geom_point()
p+ geom_point() + scale_x_continuous(expand = expansion(mult = c(0, 0))) + scale_y_continuous(expand = expansion(mult = c(0, 0)))
See ?expansion()
for more details.