Effect of `export PATH' not pemanent?

[Rodolfo Medina <rodolfo.medina@gmail.com>]

Hi.

I do:

$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games

, then

$ export PATH="$PATH:/sbin"
$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games:/sbin

and everything all right; but the effect of `export PATH'
is not permanent: after a while, or in another terminal, again:

$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games

. How to make the effect of `export PATH' permanent,
and why is it not?

Thanks,
Rodolfo