Since Lubuntu dropped Xscreensaver I'm finding this quite useful:

https://launchpad.net/caffeine

I just grab the latest .deb and install with 'gdebi'. Then you can adjust preferences in Menu > Preferences:

2013-06-29-120001_1366x768_scrot.jpg

It ain't perfect but the manual usage is good enough

It appears as a tea cup between the volume and network icons in the panel.