On some pages your visitors will want to click on the same link several times in order to reload some random-generated or very often changing content.
In this case, page caching can easily be prevented if you generate a random number/string and concatenate it to the link as a dummy parameter:
<a href=mypage.php?randomnr=<? echo $rnr; ?>>
So the link will always be different.
I use this trick on
www.stressnetz.com/glueckskekse/glueckskekse.php (the page is in german) where you can click on "another one" / "noch einen".