Use the crop package by Melchior Franz.
If you've used the geometry package to specify a custom paper size for your thesis (see the tip on Custom margins and paper sizes), there remains the problem that you will generally only have an A4 printer at your disposal, so if you want to print out (parts of) the document for proofreading, you somehow need to print the custom pages to standard A4 pages, preferably in an aesthetically pleasing way.
The crop package will do this for you, and can additionally print things such as surrounding frames, corner alignment marks, and absolute page counts.
As with geometry, it will pay to print out the package documentation and really work your way through it in order to see how you can best use the package in your own circumstances.
In the preamble:
\usepackage[a4,cam,center]{crop} \crop[font=\upshape\mdseries\small\textsf]
This package must load after the geometry package.