How to make a document occupy more pages?

These are some additional ideas, assuming that you do not need to abide by a standard layout.

Use something like the tufte-book class (it has a very wide right margin, you can make it a bit wider as well) and it still looks good.
Have full page epigraphs to separate chapters from other text and possible full page pictures. (Tufte does that at the beginning of his books).
Index almost any word you can think of.
KILL justification. All the document must be typeset ragged right.
Adjust lists and all vertical spacing.
Increase the interword spacing.
Increase the spacing after stops.
Have all sectioning at larger sized fonts.
Section the document down to paragraph (and modify) the sectioning command to start on a new line rather than as a block.
Use toc levels 7
Input chapter 3 again as the chapter before last and if anyone notices blame it on the typographical devil or the TeX engine.
If you have any computer code, include it as an appendix
Have a colophon explaining the use of LaTeX to typeset the thesis and import the source and the log as a listing.

Not yet mentioned has been the role of choosing the "right" font:

Choose a wide, but still aesthetically pleasant font (CM is pretty narrow). In my experience this can extend the number of pages by up to 20 percent. Take a look at The LaTeX Font Catalogue.
Do not use microtype, as its better spacing often reduces the number of pages (around five to ten percent).

Some additional ideas:

Replace inline lists of thoughts -- especially those in which each item contains only a handful of words -- with bulleted and/or numbered lists.
- Use nested itemize environments to further break down lists of any length into sub-lists.
- This also applies to enumerate environments.
Increase the left- and right-hand indents of "bulleted" and numbered lists as well as the offset between the "bullet"/number and the hanging paragraph.
Increase the indentation of the first lines of each paragraph, e.g., with
```
\setlength\parindent{2.5em} % you don't want your deviousness to be too obvious...
```
and also increase the spacing between paragraphs, e.g., with
```
\setlength\parskip{1.5\baselineskip} 
```

Related