Prevent Automatic Installation of iOS Update
For iOS 7 -> 8

You do not receive the message again, but the update is available through the Settings app. 

Device restarts do not cause update installation. 

Source: my wife's iPhone 5S, which went weeks and multiple reboots without installing 8.4

For iOS 9

I believe iOS 9 will prompt you daily (I upgraded immediately, but I am basing this on the comments below). However, rebooting will still not cause the update to install.