This broad answer is perhaps sufficient for a vague question.
Here is a summary of the parameters (lengths/dimensions and control sequences) that affect float placement or layout, as given by the layouts
package documentation (p 21 onward) and some extracts from the source2e
documentation:
Where floats may appear on a page and how many appear there are specified by the following float placement parameters. The numbers are named like counters so the user can set them with the ordinary counter-setting commands:
\topnumber
: Number of floats allowed at the t
op of a page (default is 2
);
\bottomnumber
: Number of floats allowed at the b
ottom of a page (default is 1
);
\totalnumber
: The total number of floats allowed per page; this includes in-text floats (default is 3
);
\topfraction
: The fraction of \textheight
t
op floats may occupy on a page (default is 0.7
);
\bottomfraction
: The fraction of \textheight
b
ottom floats may occupy on a page (default is 0.3
); and
\textfraction
: The minimum fraction of a page that must contain text (default is 0.2
).

Other page style parameters:
\floatsep
: Space left between floats (default is 12pt plus 2pt minus 2pt
);
\textfloatsep
: Space between last top float or first bottom float and the text (default is 20pt plus 2pt minus 4pt
);
\topfigrule
: Command to place rule (or whatever) between floats at top of page and text. Executed in inner vertical mode right before the \textfloatsep
skip separating the
oats from the text. Must occupy zero vertical space;
\botfigrule
: Same as \topfigrule
, but put after the \textfloatsep
skip separating text from the floats at bottom of page;
\intextsep
: Space left on top and bottom of an in-text float (default is 12pt plus 2pt minus 2pt
).

You can set any of these at the beginning of your document, and they will have a global effect from that moment onward, unless there is some command or environment explicitly resetting it.
To see what your settings for any of these parameters are, use \the...
. For example, to see the value of \floatsep
, use \the\floatsep
.
\floatsep
. What do you mean withheadline
? Maybe captions?\\
in a math environment, this introduces extra vertical space after the environment and naturally cannot be dealt with globally. Or if you use thecenter
environment in figures instead of\centering
you introduce extra vertical space.savetrees
gets rid of a lot of vertical space.