What are proper EEG frequency bands and electrode placements for imagined speech?
A study by D’Zmura et al. (2009) in which two syllables were spoken in imagination showed that imagined speech information was present in EEG alpha, beta and theta bands. The beta band (13-18 Hz) proved most informative. The most informative electrodes were located mainly near the top of the head (vertex) where electromyographic artifacts had least influence.

They used various signal processing steps (standard low-pass filtering to remove 60 Hz line artifact, DC filtering, signal enveloping etc.) to clean up their signal. Please refer to the reference list for specific signal processing details.  

References
 - D’Zmura et al. Lecture notes in computer science 2009;40-8. HCI International
 - Brigham &amp; Kumar, iCBBE 2010