Please note: As of 11/4/2011 I am in India for my Sister's wedding, so content for the next few weeks may be light.
Trying to get information from Planetary Citizens usually a hit or miss affair. (And not simply because they might open fire on interlopers.) On one hand, they're faces simply aren't human. Many of the expressions they do have are subtle or unknown to humans,or are accompanied by sounds that h. Sapiens can't hear. On the other, they have a range of body language even more diverse than humans, and a number of tics that can reveal quite a lot.
Citizen main eyes don't notably blink - they have a clear membrane and multiple semi-transparent inner lid that slide into place. The number used depends on the light level, but they are adapted to rather bright enviroments. Citizens don't notably smile or frown the way a human would either.
Since the upper arms are the most used, humans are capable of replicating many Citizen hand gestures. Convergent evolution means that in some cases, humans and aliens indicate things the same way - pointng at something universally indicates a point of interest.
Many actions have a positive and negative form in Citizen nonverbal communication. For example, if the Citizen holds its hand with the thumb meeting the last fingers and the other two in a V shape (akin to the human "peace sign") and moves their arm in an arc like waving, that is a greeting - "hello". The same movement with a closed fist is the opposite - "good bye." Touching two fingers to their opposite shoulder is an emphatic greeting - "Nice to see you/come here/ Come here, we have something to share". Making such a gesture with a closed fist means "I have nothing to say to you" or "piss off".
Of course, the most common gesture for humans to see is the citizen shrug. The upper arms scissor across each other, palms upwards -taken by humans as "I don't know/can't understand" As such, it comes up quite often with matters of vocabulary or places the citizen is unaware of. As a point of etiquette, its generally best a human not replicate this movement, as its in fact, a rather minor curse. Palms downward is the alien analog of "Amen" or "so it is the planet's will" meaning upwards more accurately translates to "Not of this world/ what is this accursed thing?"
Another common gesture with the V or Fist dichotomy is to touch the hand to either their nose, or antenna base. This is an indication of noticing or failing to notice something nearby (nose) or far away (antenna).
Although not an intentional gesture, a citizens antennas can sometimes indicate their mood. Most easily visible is rapidly moving around, searching for something, which indicates distress or fear of someting.