I'm writing a short document of (3-4 pages), and try to condense it into the allowed space.
Putting \looseness=-1 at the end a paragraph helps quite a lot. Is there a way to tell LaTeX to treat every paragraph as if there is a \looseness=-1 at its end, or must I put the command after each and every paragraph by myself?
At the end? Rather at the beginning. And generally:
\everypar{\looseness=-1}
However it does not mean that every paragraph will be one line shorter.
\everypar in this manner might very well overrule some "desirable" settings established in basic latex and various document classes and packages. for example, check its effect in itemized and enumerated lists. (i haven't tried it, but a quick scan of latex.ltx shows that \everypar is used extensively.)
Commented
Nov 25, 2013 at 19:50
\everypar=\expandafter{\the\everypar\loosness=-1 }. One can put it at the end of each paragraph by redefining \par, but LaTeX also uses \par very extensively. Finally, one can simply set \linepenalty=1000 (or larger). This adds a penalty for each line and so TeX tries to reduce the number of lines. It is not reset with each paragraph, so it only needs to be set once. Basic LaTeX does not touch \linepenalty except to set the default value (10).
Dan's suggestion in a comment to Przemysław Scherwentke's answer deserves more visibility:
Finally, one can simply set
\linepenalty=1000(or larger). This adds a penalty for each line and so TeX tries to reduce the number of lines. It is not reset with each paragraph, so it only needs to be set once. Basic LaTeX does not touch\linepenaltyexcept to set the default value (10).