There are two trustworthy tools for adjusting the dimensions and position of the printed material on the page are geometry and the zwpagelayout packages; a very wide range of adjustments of the layout may be relatively straightforwardly programmed with either, 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 both of geometry and of zwpagelayout is rather overwhelming, and learning all of of either package’s capabilities is likely to be more than you ever need. The vmargin package is somewhat simpler to use: it has a canned set of paper sizes (a superset of that provided in LaTeX 2e), provision for custom paper, margin adjustments and provision for two-sided printing.
This answer last edited: 2011-12-12