Arrows at right angles on a tikzpicture matrix

Instead of using an edge, you can use a to operation (which has to be used with \draw, not \path). That will give you very fine control over how the nodes or coordinates are connected. You can define any arbitrary path using to path=<path definition>, in which (\tikztostart), (\tikztotarget) and (\tikztonodes) will hold the start and end of the path and the nodes you define with the to operation. If you define a style containing to path={(\tikztostart) -- ++(#1,0pt) \tikztonodes |- (\tikztotarget) }, you can pass an argument to the style and the to path will then draw a horizontal line for a specified distance, place the nodes on that line, and then draw an orthogonal line to the target.

I've also changed your code to use the \tikzset{<style name>/.style={<options>}} syntax instead of the obsolete \tikzstyle command.

\documentclass[a4paper,10pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usetikzlibrary{matrix,shapes,arrows,positioning,chains}

\begin{document}

% Define block styles
\tikzset{
    desicion/.style={
        diamond,
        draw,
        text width=3em,
        text badly centered,
        inner sep=0pt
    },
    block/.style={
        rectangle,
        draw,
        text width=10em,
        text centered,
        rounded corners
    },
    cloud/.style={
        draw,
        ellipse,
        minimum height=2em
    },
    descr/.style={
        fill=white,
        inner sep=2.5pt
    },
    connector/.style={
     -latex,
     font=\scriptsize
    },
    rectangle connector/.style={
        connector,
        to path={(\tikztostart) -- ++(#1,0pt) \tikztonodes |- (\tikztotarget) },
        pos=0.5
    },
    rectangle connector/.default=-2cm,
    straight connector/.style={
        connector,
        to path=--(\tikztotarget) \tikztonodes
    }
}

\begin{tikzpicture}
\matrix (m)[matrix of nodes, column  sep=2cm,row  sep=8mm, align=center, nodes={rectangle,draw, anchor=center} ]{
\node [block] {A. Campos [1;14]}; & \\
\node [block] {B. Bineado [1;4]}; & \\
\node [cloud] {call \texttt{analisis\_campo}}; & \\
\node [block] {C. Modelos [1;2]}; & \\
\node [block] {D. Extinción [1;31]}; &  \node [block] {E. Fracción de binarias [1;2]}; \\
\node [cloud] {call \texttt{analisis\_modelo}}; & \\
\node [cloud] {call \texttt{analisis\_final}}; & \node [desicion] {\scriptsize{Fin E}}; \\
\node [desicion] {\scriptsize{Fin D}}; & \\
\node [desicion] {\scriptsize{Fin C}}; & \\
\node [desicion] {\scriptsize{Fin B}}; & \\
\node [desicion] {\scriptsize{Fin A}}; & \\
\node [block] {Stop}; & \\
 };
\path [>=latex,->] (m-1-1) edge (m-2-1);
\path [>=latex,->] (m-2-1) edge (m-3-1);
\path [>=latex,->] (m-3-1) edge (m-4-1);
\path [>=latex,->] (m-4-1) edge (m-5-1);
\path [>=latex,->,dashed] (m-5-1) edge node[auto] {\scriptsize{MODEL}} (m-5-2);
\draw [>=latex,->] (m-5-2) |- (m-6-1);
\path [>=latex,->] (m-5-1) edge (m-6-1);
\path [>=latex,->] (m-6-1) edge (m-7-1);
\path [>=latex,->,dashed] (m-7-1) edge node[auto] {\scriptsize{MODEL}} (m-7-2);
\path [>=latex,->] (m-7-1) edge (m-8-1);
\draw [rectangle connector=2.5cm] (m-7-2) to node[descr] {No} (m-5-2);
\draw [rectangle connector=-3cm] (m-8-1) to  node[descr] {No} (m-5-1);
\draw [rectangle connector=-4cm,] (m-9-1) to node[descr] {No} (m-4-1);
\draw [rectangle connector=-5cm] (m-10-1) to  node[descr] {No} (m-2-1);
\draw [rectangle connector=-6cm] (m-11-1) to node[descr] {No} (m-1-1);
\draw [connector] (m-7-2) |- node[descr, pos=0.25] {Sí} (m-8-1);
\draw [straight connector] (m-8-1) to node[descr] {Sí} (m-9-1);
\draw [straight connector] (m-9-1) to node[descr] {Sí} (m-10-1);
\draw [straight connector] (m-10-1) to node [descr] {Sí} (m-11-1);
\draw [straight connector] (m-11-1) to node [descr] {Sí} (m-12-1) ;
\end{tikzpicture}

\end{document}

If you're using tikz v3.0

The syntax has changed a little for v3.0; the above code can be adapted to

% arara: pdflatex
% !arara: indent: {overwrite: yes}
\documentclass[a4paper,10pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usetikzlibrary{matrix,shapes,arrows,positioning,chains}

\begin{document}

% Define block styles
\tikzset{
    desicion/.style={
        diamond,
        draw,
        text width=3em,
        text badly centered,
        inner sep=0pt
    },
    block/.style={
        rectangle,
        draw,
        text width=10em,
        text centered,
        rounded corners
    },
    cloud/.style={
        draw,
        ellipse,
        minimum height=2em
    },
    descr/.style={
        fill=white,
        inner sep=2.5pt
    },
    connector/.style={
        -latex,
        font=\scriptsize
    },
    rectangle connector/.style={
        connector,
        to path={(\tikztostart) -- ++(#1,0pt) \tikztonodes |- (\tikztotarget) },
        pos=0.5
    },
    rectangle connector/.default=-2cm,
    straight connector/.style={
        connector,
        to path=--(\tikztotarget) \tikztonodes
    }
}

\begin{tikzpicture}
    \matrix (m)[matrix of nodes, column  sep=2cm,row  sep=8mm, align=center, nodes={rectangle,draw, anchor=center} ]{
        |[block]| {A. Campos [1;14]}               &                                             \\
        |[block]| {B. Bineado [1;4]}               &                                             \\
        |[cloud]| {call \texttt{analisis\_campo}}  &                                             \\
        |[block]| {C. Modelos [1;2]}               &                                             \\
        |[block]| {D. Extinción [1;31]}            & |[block]| {E. Fracción de binarias [1;2]}   \\
        |[cloud]| {call \texttt{analisis\_modelo}} &                                             \\
        |[cloud]| {call \texttt{analisis\_final}}  & |[desicion]| {\scriptsize{Fin E}}           \\
        |[desicion]| {\scriptsize{Fin D}}          &                                             \\
        |[desicion]| {\scriptsize{Fin C}}          &                                             \\
        |[desicion]| {\scriptsize{Fin B}}          &                                             \\
        |[desicion]| {\scriptsize{Fin A}}          &                                             \\
        |[block]| {Stop}                           &                                             \\
    };
    \path [>=latex,->] (m-1-1) edge (m-2-1);
    \path [>=latex,->] (m-2-1) edge (m-3-1);
    \path [>=latex,->] (m-3-1) edge (m-4-1);
    \path [>=latex,->] (m-4-1) edge (m-5-1);
    \path [>=latex,->,dashed] (m-5-1) edge node[auto] {\scriptsize{MODEL}} (m-5-2);
    \draw [>=latex,->] (m-5-2) |- (m-6-1);
    \path [>=latex,->] (m-5-1) edge (m-6-1);
    \path [>=latex,->] (m-6-1) edge (m-7-1);
    \path [>=latex,->,dashed] (m-7-1) edge node[auto] {\scriptsize{MODEL}} (m-7-2);
    \path [>=latex,->] (m-7-1) edge (m-8-1);
    \draw [rectangle connector=2.5cm] (m-7-2) to node[descr] {No} (m-5-2);
    \draw [rectangle connector=-3cm] (m-8-1) to  node[descr] {No} (m-5-1);
    \draw [rectangle connector=-4cm,] (m-9-1) to node[descr] {No} (m-4-1);
    \draw [rectangle connector=-5cm] (m-10-1) to  node[descr] {No} (m-2-1);
    \draw [rectangle connector=-6cm] (m-11-1) to node[descr] {No} (m-1-1);
    \draw [connector] (m-7-2) |- node[descr, pos=0.25] {Sí} (m-8-1);
    \draw [straight connector] (m-8-1) to node[descr] {Sí} (m-9-1);
    \draw [straight connector] (m-9-1) to node[descr] {Sí} (m-10-1);
    \draw [straight connector] (m-10-1) to node [descr] {Sí} (m-11-1);
    \draw [straight connector] (m-11-1) to node [descr] {Sí} (m-12-1) ;
\end{tikzpicture}

\end{document}

Jake's answer is fine. I give another possiblity

\draw [>=latex,->,font=\scriptsize] (m-8-1) -- +(-3,0) |-  node[pos=.25,descr] {No} (m-5-1);
\draw [>=latex,->,font=\scriptsize] (m-9-1) -- +(-4,0) |-  node[pos=.25,descr] {No} (m-4-1);
\draw [>=latex,->,font=\scriptsize] (m-10-1)-- +(-5,0) |-  node[pos=.25,descr] {No} (m-2-1);
\draw [>=latex,->,font=\scriptsize] (m-11-1)-- +(-6,0) |-  node[pos=.25,descr] {No} (m-1-1);