I've already been explaining in my previous post about application pools how to save on page file by reusing application pools.
When you need to release page file size and memory consumption, you can run iisreset. This will kill each application pool for each site and therefore release the memory (untill the site is revisited).
Joel Oleson is describing an alternative way to release the application pool and cache that we don't need without the 7-seconds-long iisreset. Below quoting from Joel's blog:
cscript c:\windows\system32\iisapp.vbs /a "%SharePointAppPool%" /r
Where %SharePointDefaultAppPool% is the app pool you are wanting to cycle.