How can I decrease spaces between equations?

I would recommend the following measures:

(1) Instead of \renewcommand\baselinestretch{1.5}, load the setspace package with the nodisplayskipstretch option, and then execute the command setstretch{1.5} in your document's preamble:

...
\usepackage[nodisplayskipstretch]{setspace}
\setstretch{1.5}

This way, you won't experience the excessive spacing between lines of text and floating objects. Also, material in footnotes will remain single-spaced.

(2) When you have consecutive equations, don't use separate equation environments but instead use the gather environment provided by the amsmath package. (If the equations should be aligned along, say, their equal signs, use the align environment instead of the gather environment, using the ampersand, &, character as the alignment point.) Two consecutive equations would then be entered as

\begin{gather}
1 + 2 + 3 = 6 \\
2 + 4 + 6 = 12
\end{gather}  

LaTeX uses \abovedisplayskip and \belowdisplayskip for the spacing above/below equations. There's also a short version of the former two commands for paragraphs ending/beginning with shorter lines. Here's an example showing the difference:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
Text text text text text text text text text text
\begin{equation}
1 + 2 + 3 = 6
\end{equation}  
Text text text text text text text text text text
\begin{align}
1 + 2 + 3 &= 6 \\
1 + 2 + 3 &= 6
\end{align}

\setlength{\belowdisplayskip}{0pt} \setlength{\belowdisplayshortskip}{0pt}
\setlength{\abovedisplayskip}{0pt} \setlength{\abovedisplayshortskip}{0pt}

Text text text text text text text text text text
\begin{equation}
1 + 2 + 3 = 6
\end{equation}  
Text text text text text text text text text text
\begin{align}
1 + 2 + 3 &= 6 \\
1 + 2 + 3 &= 6
\end{align}
\end{document}​

The same effect is obtained with/without a modified \baselinestretch - it is just clearer when viewed without that set. Of course, you can modify these lengths as needed.

Edit: Heed @egreg's suggestion and/or @Mico's answer regarding two equations

you should use package setspace for the vertical spacing. The distance between two equations is set by the short skips!

\documentclass[12pt,a4paper]{mwrep}    
\usepackage{setspace}\onehalfspacing
\AtBeginDocument{%
  \addtolength\abovedisplayskip{-0.5\baselineskip}%
  \addtolength\belowdisplayskip{-0.5\baselineskip}%
%  \addtolength\abovedisplayshortskip{-0.5\baselineskip}%
%  \addtolength\belowdisplayshortskip{-0.5\baselineskip}%
}

\begin{document}
\chapter{Test}
Text text text text text text text text text text      
\begin{equation}
1 + 2 + 3 = 6
\end{equation}  
Text text text text text text text text text text  
\begin{equation}
1 + 2 + 3 = 6
\end{equation}
\begin{equation}
1 + 2 + 3 = 6
\end{equation}
some nmore text\par
some more text

\end{document}