Headers are also particularly vertical. That is, they consist of a bunch of lines that push your writing down. How you format them (staying within the guidelines of course) can make a pretty big difference.

Extending your paper

If you're trying to extend your paper (making it appear longer than it would normally be) you're going to want to increase your content. Is there anything else that you can think to add that isn't just repeating yourself over and over? Sometimes there isn't. If this is the case, move onto line breaks. In this case, you're going to want to first target paragraphs who have both

a last line that takes up most of the page horizontally

several lines that reach almost to the margin

Of course, this can work in paragraphs that aren't like this, but it'll take more effort. Start by looking at the sentences on the "full" lines. 

Are there any places where you're unclear or glossed over something? If making it clearer would be longer, replace it. 

Are there places where you can replace a weaker verb, noun, adjective, etc with something stronger (and longer)? Do it. 

Would adding a simile or metaphor help add more texture to your writing? Add it. 

Also keep in mind the space individual letters take up. For example, the letter i takes up almost no space compared to the letter m. In some cases, words with the same number of letters can be different lengths based on how they're spelled. Use this to your advantage. 

The idea is that you're making your essay longer, but you're also strengthening your writing - this will help with your grade. 

The next thing to do is to expand any contractions into their full form - this is a good practice anyway, and will help expand your paper without adding new content, or worse, making it bloated. Also, expand any dates. Your subject matter died on Dec. 12 1987? Now he died December 12, 1987. ESPECIALLY do this if you've listed any dates like 12/12/87. Also, spell out any numbers: your subject matter wrote twelve books, not 12. This is not a math textbook. If applicable, expand titles when people are first introduced. These changes may be small, but either they will create new lines, or they will help disguise some of your whitespace, making it appear that there's more content. Both are good.

The last thing to do is fiddle with your headers. Ideally, you've already hit your page count. This is just to make it look like you weren't struggling to put content on the page. Basically, if there's any optional lines to your header (teacher's name, or class name are common ones for me), add them. You're following the guidelines as they're intended (sort of) so you're not going to get called out on it. Best of all, it will push your paper down a little farther on the page, filling it out. In the same vein, make sure you've got any block quotes properly formatted - they're going to have narrower margins than the rest of your paper.

Compressing your paper

You've got all this great content but not enough space? Don't worry. You probably won't have to cut as much as you think. 

The first step is a little different than with expanding your paper. Go through the whole thing and see what information isn't pulling its weight. Redundant? Convoluted? Irrelevant? Cut or simplify accordingly. Depending on how you've written your paper, this could range from minor edits to something that changes the length by several lines.

Next thing is to identify paragraphs that fit almost the opposite criteria as I described above. Here, the ideal paragraph is one that has

Lots of shorter lines

a last line that doesn't take up much of the page.

Now you're going to clean up your writing on a smaller scale. To start, find words at the start of lines that are longer, and see what shorter words convey the same or more specific meaning. Replace it. Also see if there are ways to take longer and meandering sentences and cut the parts that are more extraneous. This will have the added benifit of making your work clean and well-crafted. 

Now, find filler words. Two common ones are "very," "that," and "just." If your sentence means the same thing without them and is gramatically accurate, cut them. They're not doing you any favors. Also, except for when you're introducing someone, you often only need their last name. Cut useless instances of their given name. 

Finally, focus on headers. Some teachers don't care much about what info you include. If there are any optional details, leave them off. At a bare minimum, you need your name, and often your date, but some teachers may not care if you put their name, for example. 

Good luck with your papers!

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Academic Stuff taglist: @candlemouse