Modern versions of OS X ship with excellent power saving features such as “quit idle apps” which improves your battery life. For case where you want to keep your idle apps open there is
defaults write -g NSDisableAutomaticTermination -bool TRUE 
More information and how to edit this in per-app basis see AlignSoft’s post.
: If you don’t know why this would be useful you don’t need it — and probably wont actually need it even when you think you do Just let the OS do what is best.