An alternative to theme_classic() is the theme that comes with the cowplot package, theme_cowplot() (loaded automatically with the package). It looks similar to theme_classic(), with a few subtle differences. Most importantly, the default label sizes are larger, so the resulting figures can be used in publications without further modifications needed (in particular if you save them with save_plot() instead of ggsave()). Also, the background is transparent, not white, which may be useful if you want to edit the figure in illustrator. Finally, faceted plots look better, in my opinion.
Example:
library(cowplot)
a <- seq(1,20)
b <- a^0.25
df <- as.data.frame(cbind(a,b))
p <- ggplot(df, aes(x = a, y = b)) + geom_point()
save_plot('plot.png', p) # alternative to ggsave, with default settings that work well with the theme
This is what the file plot.png produced by this code looks like:
Disclaimer: I'm the package author.