You need to install MATCHIT only once, but you must load it prior to each use. You can do this at the R prompt:
> library(MatchIt)
Alternatively, you can specify R to load MATCHIT automatically at launch by editing the Rprofile file located in the R program subdirectory, e.g. C:/R/rw2011/etc/, for Windows systems or the .Rprofile file located in the home directory for Unix/Linux and Mac OS X systems, and adding this line:
options(defaultPackages = c(getOption("defaultPackages"), "MatchIt"))
For this change to take effect, you need to restart R.