Is it possible to create a header in LaTeX containing my usual preamble, functioning similar to a header in C? Specifically, can I write a text document (or something similar) with the preamble that I usually use, and type something in my documents' preambles that effectively pastes this document there? In particular, so that I can change this document, and have all of my TeX documents' preambles effectively updated? (I keep making small changes in one document, and updating all the others is getting rather troublesome...) Thanks!
If I understand your question correct, you want a file containing your preamble and you want to include it in every document. Then just put the parts of the preamble that are not document specific into a file and save that file somewhere in the LaTeX search path, e. g.
You can then
For a more advanced technique see also the question How to make a standard preamble into a package.
That's a pretty basic mechanism in (La)TeX which you can also use to "separate out" other parts of your document.
Simply paste any number of lines into a new text file (say,
in the place where the lines used to be (note you can leave out the