Check for installed packages before running install.packages()

try: require("xtable") or "xtable" %in% rownames(installed.packages())

If you want to do it as simply as possible:

packages <- c("ggplot2", "dplyr", "Hmisc", "lme4", "arm", "lattice", "lavaan")

install.packages(setdiff(packages, rownames(installed.packages())))  

Replace the packages listed on the first line by those needed to run your code, and voilà!

Note: Edited to remove conditional wrapper thanks to Artem's comment below.

This is a function I often used to check for a package, install it otherwise and load again:

pkgTest <- function(x)
  {
    if (!require(x,character.only = TRUE))
    {
      install.packages(x,dep=TRUE)
        if(!require(x,character.only = TRUE)) stop("Package not found")
    }
  }

Works like pkgTest("xtable"). It only works if the mirror is set though, but you could enter that in the require calls.