The ‘ultimate’ tool for adjusting the dimensions and position of the printed material on the page is the geometry package; a very wide range of adjustments of the layout may be relatively straightforwardly programmed, and package documentation is good and comprehensive.
As is usual, users of the memoir class have built-in facilities for this task, and users of the KOMA-script classes are recommended to use an alternative package, typearea. In either case it is difficult to argue that users should go for geometry: both alternatives are good.
The documentation of geometry is a bit overwhelming, and learning all its capabilities may be more than you ever need. Somewhat simpler to use is the vmargin package, which has a canned set of paper sizes (a superset of that provided in LaTeX2e), provision for custom paper, margin adjustments and provision for two-sided printing.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=marginpkgs