I believe that ~/bin is already part of the shell path. If you put rar in a folder called bin in your home directory then it will be found by the shell. (You may have to run the "rehash" command if the terminal is already open, although I don't think this command exists in bash.)
I don't recommend dropping a bunch of unix binaries into your Applications folder. It's not really the appropriate place for them. /usr/local/bin or ~/bin is a wiser choice.
That said, you can add other directories to your $path by adding a command like the following to a file called .bashrc in your home directory:
export PATH=/Applications:$PATH