Precise placement of arrow anchors

You can specify the horizontal and vertical position in terms of nodes.

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
  \node[rectangle, draw] (a) at (0,0) {Some rather long text};
  \node[rectangle, draw] (b) at (.5,-2) {Even more text};
  \draw[->] (b|-a.south) -- (b);
\end{tikzpicture}
\end{document}

Or, if you want the arrow to be in the center,

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
  \node[rectangle, draw] (a) at (0,0) {Some rather long text};
  \node[rectangle, draw] (b) at (.5,-2) {Even more text};
  \path (a) -- (b) coordinate[midway] (aux);
  \draw[->] (a.south -| aux) -- (b.north -| aux);
\end{tikzpicture}
\end{document}