A new study published in Astrobiology suggests how extraterrestrials might be trying to contact us.

Arecibo Radio Telescope
The study suggests that budget-conscious extraterrestrial scientists would probably be more selective in their broadcasts, limiting their transmissions to frequencies with less astronomical background noise, and sending only short pulses, rather than an energy-hogging continuous broadcasts.
If extraterrestrials attempting to contact us are implementing a more conservative approach than scientists have previously assumed, then adjustments to the “listening” process would be required to prevent those messages from going unnoticed.
This topic was discussed on the July 26, 2010 episode of Open Minds Radio.