Creating a function

Another way is to build a layer via a function:

geom_lm <- function(formula = y ~ x, colour = alpha("steelblue", 0.5), 
                    linewidth = 2, ...)  {
  geom_smooth(formula = formula, se = FALSE, method = "lm", colour = colour,
    linewidth = linewidth, ...)
}

Use the layer in the plot:

ggplot(mpg, aes(displ, 1 / hwy)) + 
  geom_point() + 
  geom_lm(y ~ poly(x, 2), linewidth = 1, colour = "red")