Instructions on using a can of compressed air to dust off optical surfaces, 
from experienced scope user Jay Faircloth:

The primary 'danger' often mentioned in discussions on the use of compressed air
cans is that of getting the liquid propellant (the can's 'spittle'so to speak, 
which is an organic liquid, not air) onto the glass. It is said that say this 
can be a real hassle to remove. My personal feeling (and experience) is that with 
a few precautions, this can be avoided. First - don't agitate the can. For some 
reason we humans have a tendency to shake an aerosol can like it was spray paint 
when we first pick it up. It doesn't matter if it's spray paint, whipped cream, 
WD-40 or compressed air...we tend to shake it. Don't do that!! Pick it up as 
gently as possible - pretend it's nitroglycerin! Next, keep the can erect - 
straight up & down - meaning don't tilt it under any circumstance. Doing so can 
allow the liquid propellant to get into the interior 'pickup' tube. Next, only 
'fire' it in short bursts....2 or 3 seconds at the most. Your first couple of 
'shots' should be _away_ from the target, to ensure that any of the liquid that 
might've gotten into the tube, is first cleared. Then after 4 or 5 bursts at the 
'target' (corrector lens or eyepiece), let the can 'rest' 10 seconds or so.  With
long bursts, vacuuming or pressure differentials are created in the can 
(thermodynamics become part of the formula) and as a result, the liquid propellant 
can get picked up by the 'pickup' tube. 

Another particular caution is this: if the can even _starts_ to feel cold in your 
hand, STOP! Don't use it any more until it re-warms. Actually, if you're spraying in
short bursts it should never do that....so that's a secondary 'warning' that you
should change your ways!

Lastly, don't try to get every last iota out of the can. At the point you can
first tell that the pressure is falling off, that's the point to stop using it on 
your optical glass. This might at first seem wasteful (despite how cheap it really 
is) but err on the side of caution. Save it for other uses if you like, but for 
your optics, get a new can! The liquid propellant is again the culprit here...if 
you get down 'too low' in the can, the chance of picking it up is greater.