This code generates simple but elegant slides and avoids the clutter of many more sophisticated styles. Its main advantage, however, is that it allows to directly recycle the source code of an article (including section commands etc.) .

\documentclass[20pt,a4paper,landscape,titlepage,endnotes]{ext article}
\setlength{\topmargin}{-1.04cm}
\setlength{\textheight}{15cm}
\setlength{\oddsidemargin}{-0.04cm}
\setlength{\evensidemargin}{-0.04cm}
\setlength{\textwidth}{24.7cm}
\setlength{\parindent}{0cm}
\setlength{\parskip}{.2cm}
\newcommand{\longseminar}[1]{#1} % for slides that are only presented in long seminars
% \newcommand{\longseminar}[1]{\!}
\newcommand{\new}{\newpage}
% \newcommand{\details}[1]{#1}
\newcommand{\details}[1]{}
\usepackage{ulem}
\newcommand{\striketextthrough}[1]{\sout{#1}}
\normalem % to disable ulem package after this command definition, because ulem also changes \emph etc.
\usepackage{fancyhdr}
\pagestyle{fancy} % headings, plain, empty, headings, myheadings
\renewcommand{\headrulewidth}{0.4pt}
\renewcommand{\headrulewidth}{0pt}
\renewcommand{\footrulewidth}{0.4pt} % text for footers and headers defined in document
\fancypagestyle{plain}{\fancyfoot{}\fancyhead{}\renewcommand{\headrulewidth}{0pt}\renewcommand{\footrulewidth}{0pt}}
\newcommand{\origsection}{}
\let\origsection=\section
\renewcommand{\section}[1]{\newpage\origsection*{#1}\markboth{#1}{}}
%\newpage\pagenumbering{arabic}\renewcommand{\thepage}{\thesection--\arabic{page}}
\newcommand{\origsubsection}{}
\let\origsubsection=\subsection
%\renewcommand{\subsection}[1]{\newpage\origsubsection*{#1}\markright{#1}{}}
\renewcommand{\subsection}[1]{\origsubsection*{#1}\markright{#1}{}}
\newcommand{\origsubsubsection}{}
\let\origsubsubsection=\subsubsection
\renewcommand{\subsubsection}[1]{\origsubsubsection*{#1}}
\newcommand{\origitem}{}
\let\origitem=\item
\renewcommand{\item}{\vspace{-.4cm}\origitem[$\rhd$]}
\renewcommand{\item}{\origitem}
\usepackage[font=footnotesize]{caption}
\newcommand{\origcaption}{}
\let\origcaption=\caption
\renewcommand{\caption}[1]{\origcaption*{#1}}
\renewcommand{\caption}{\origcaption}
\makeatletter
\usepackage{type1cm}
\renewcommand\normalsize{%
\@setfontsize\normalsize{25pt}{29pt}%
\abovedisplayskip 12\p@ \@plus3\p@ \@minus7\p@
\abovedisplayshortskip \z@ \@plus3\p@
\belowdisplayshortskip 6.5\p@ \@plus3.5\p@ \@minus3\p@
\belowdisplayskip \abovedisplayskip
\let\@listi\@listI}
\normalsize
\makeatother
\usepackage{color}
% … here add additional, standard commands, settings, packages, ...
% Workaround to use hyperref and landscape slides
% Remove if you are using portrait slides
\makeatletter
\def\special@paper{297mm,210mm}
\makeatother % this workaround has to come after a hyperref call
\usepackage{mathpazo}