So I had one answer (which is preserved below for posterity) but I think I understand the question a little better now. What you want is to write \input{foo} and have LaTeX know where to search for foo. Martin's answer offers one way to do this. Here are some others.
One way is to make a directory in your local texmf tree and put files you want to use in there. So (assuming a uni system) this will be something like ~/texmf/ Make a subdirectory e.g. ~/texmf/tex/latex/commonstuff/ and put your files in there. Then they will be available to \input. Be careful about name clashes, though!
If, for whatever reason, you cannot move where your files are stored, you could add that directory to TEXINPUTS which is a variable that stores the directories that tex searches for files called with \usepackage \input and similar commands.
Further reading:
I maintain that this may be overkill if the problem is simply that some files live in the folder "above" the one containing the tex file (when the ../ trick discussed below works fine)
I don't really understand the question. As far as I am aware, absolute paths work with \input? I just tried one and it worked.
Sometimes it is easier to use relative paths, but to know how to use them. For example a lot of the time I want to insert a graphic which is housed in a different folder. Let's say I have the following:
Work Document main.tex Graphics img.pdf
In main.tex I can write \includegraphics{../Graphics/img} and this finds my image: .. goes "up a level".
\CurrentFilePathto avoid having to use absolute paths.