How do others manage their footnotes?

A easier option could be clipboard:

The file with the footnotes: (for example footnotes.tex)

\documentclass{article}
\usepackage{clipboard}
\newclipboard{myfootnotes}
\begin{document}
\Copy{lore}{\footnote{This is a footnote.}}
\Copy{ipsum}{\footnote{This is a more long footnote.}}
\Copy{dolor}{\footnote{\emph{Neque porro quisquam est qui dolorem ipsum quia dolor sit amet, consectetur, adipisci velit}.}}
\end{document}

The file with the main text: (for example main.tex)

\documentclass[a5paper,12pt]{article}
\usepackage[bmargin=14cm]{geometry}
\usepackage{clipboard}
\openclipboard{myfootnotes}
\begin{document}
This is the main text with a footnote,\Paste{lore}
another footnote \Paste{ipsum}  and one Cicero quote.
\Paste{dolor}
\end{document}

Notes:

The file names are irrelevant, but obviously both files must share the name of the clipboard.

Because content of footnote.tex is copied (to a temporal myfootnotes.cpy) and pasted into main.tex, you must compile first footnote.tex and after that, compile main.tex.

The original purpose of this package is to reuse the text of one stand alone document into anothers, but if the first file is only to maintain the clipboards, alternatively you can write directly myfootnotes.cpy, including only commands with this format:

\clipboard{name}{text}

But obviously, if you use \newclipboard{myfootnotes} in any .tex file, this .cpy file will be overwritten!

The result:

Yep. This is precisely what the sepfootnotes package does, as cgnieder has remarked above:

\documentclass{article}
\usepackage{sepfootnotes}
\newfootnotes{x}
\xnotecontent{key}{content}
\begin{document}
bla bla bla\xnote{key}
\end{document}

My solution :

1.file Notes.tex:

\documentclass[8pt]{article}

\begin{document}
%<*notes004>
\footnote{La mia footnotes è questa.}
%</notes004>

\end{document}

2. in the main.tex file:

\usepackage{catchfilebetweentags}
\newcommand{\loadnote}[1]{%
    \ExecuteMetaData[notepie.tex]{notes#1}%
}

3. for example in the masterdom.tex insert

I want to explain what is the method to insert footnotes in an external file, to know it is necessary to follow the instructions\loadnote{004}

ps : I did not know the existence of this post and I wrote another post