If your monitor wakes up (almost) immediately after sleep
You may face this problem when you send your monitor to sleep. A few seconds or minutes later the monitor wakes up by itself, without any input from keyboard or mouse. In this and the next page I will let you know and try several methods to get rid of the wake problem. Please note that more causes may exist not covered here.
As for myself, since the monitor kept waking up forced by certain applications such as Firefox, instead of closing these applications each and every time I wanted my monitor to sleep, I decided to just use this small freeware called Super Sleep, that locks a monitor in sleep mode, so that no application or system setting is able to wake it up. You can use Super Sleep yourself, or keep reading.
Try first to locate possible applications that wake up your monitor, check all startup locations (even in the registry) for programs that may perform automatically tasks, etc. — but even before this, just close main applications you may have running, such as your browser or word processor, torrent clients and peer 2 peer applications, and then put your monitor to sleep, because the problem may come from such programs.
In my case Firefox is one of these offending applications. Not wanting to believe it, I disabled all Firefox addons, even disabled auto-update and crash reports — still Firefox interrupted my monitor’s sweet dreams. I need to have Firefox always open, and since the same problem is caused by other applications too, I skipped the ‘solution’ of closing my programs before activating power save mode, by letting Super Sleep keep my monitor in deep dreams safely, no matter what programs or Desktop Gadgets I have constantly working.