One way to achieve this is by modifying the .bashrc file.
Simply place the following at the end of the .bashrc file.
PS1="\n$PS1"
To explain how this works, PS1 is the variable containing what should be displayed as the prompt. All this is saying is "set PS1 to the previous contents of PS1, with a newline character prepended". Putting it in .bashrc on most distros just makes bash run it every time you open an interactive shell (but not a login shell - see Difference between Login Shell and Non-Login Shell?).
