By default, irssi comes with statusbars for "window" and "window_inact", and
in default.theme they are themed indistinguishably.  If you have multiple
visible split windows (see "An Illustrated Guide to Split Windows in
Irssi"[http://f0rked.com/articles/irssisplit] for some great info, and perhaps
back up your config and read the guide through a couple times before trying
anything fancy), and one input prompt at the bottom to rule them all, it can
get confusing to see at a glance which window will recieve your typed input.
One thing that can help this situation is to use a theme or edit your favorite
theme so that inactive windows' statusbars get a low-key color scheme, and the
active window's statusbar is more attention-grabbing.  

The theme items for statusbar backgrounds are named using the scheme
sb_[statusbar name]_bg, and the current statusbar names are available via the
/statusbar command (for example, "window" and "window_inact", above.  In
default.theme, you would add a line to the "## statusbar" section near the
bottom, changing the background on window_inact statusbars like so:

sb_window_inact_bg = "%N";

See formats.txt[http://f0rked.com/public/irssi-docs/formats.txt] for a
reference of the possible color codes and combinations available.  The above
example would set the default color on window_inact statusbars to match the
terminal's default foreground/background, which for standard white text on a
black background would attract the eye less than default.theme's blue
background.

You may find that if you take this approach, the divsion between split windows
isn't as visually distinct as you'd like, in which case you may want to add a
topicbar to all visible windows by making the standard "topic" statusbar a
window- rather than a root statusbar with the command:

/statusbar topic type window

Another option is to keep a colored background on the "window_inact"
statusbars, but keep it visually low key compared to the "hilighted" color you
choose for the "window"

In the screenshot below
[http://billnye.blinkenshell.org/irssi/putty20080818.png], there are three
visible split windows, with the active window in the bottom split.  Window
topicbars with a blue background have been used, inactive window statusbars
have a transparent background, and the active window has a red statusbar.