There are basically two possibilities, both deal with splitting into several files
- Include each file with the
\input{filename} command
- Include each file with the
\include{filename} command
The second way always adds a pagebreak.
Regardless which style you apply, I would split the large file in logically parts, such as chapters or sections, but not into subsections.
Please note, that you cannot nest \include commands, in that case, you have to use \input in deeper hierarchies.
If you want to test some parts only, you could use \includeonly command, say
\documentclass{article}
%Starting stuff
\includeonly{section1,section6}%
\begin{document}
%Something to be done before formatting etc. of solutions
\include{section1}
\include{section2}
\include{section3}
\include{section4}
\include{section5}
\include{section6}
% Other stuff here
\end{document}
This will include only the files with section1 and section6, although the other ones are given explicitly in the \include statement.
Please also have a look on the filecontents package.