http://blogs.technet.com/b/mspfe/archive/2014/02/05/freeing-memory-for-hyper-v-vms-on-windows-8-8-1.aspx

Most people will start closing desktop applications like Outlook or Word in order to free RAM, but I like my Outlook open, thank you, so I will show you a better way by using a free Sysinternals tool by Mark Russinovich, called RAMmap.

When you start RAMMap, it shows a “map” of the memory: On the top there is an “Empty” menu, so I click on “Empty Working Sets”, … press F5 to refresh. Wow, now I have more that 5GB on standby, all my apps are still working and I can start my VM(s) now.