I use RStudio or Emacs and always use the editor shortcuts available to comment regions. If this is not a possibility then you could use Paul's answer but this only works if your code is syntactically correct.
Here is another dirty way I came up with, wrap it in scan() and remove the result. It does store the comment in memory for a short while so it will probably not work with very large comments. Best still is to just put # signs in front of every line (possibly with editor shortcuts).
foo <- scan(what="character")
These are comments
These are still comments
Can also be code:
x <- 1:10
One line must be blank
rm(foo)