I put a small (M4 if I remember correctly) stainless steel nut in my Vallejo paints, you can get packs of a hundred on eBay fairly cheaply. I found the ball bearings drop into the neck of the bottle as you dispense the paint, effectively blocking the nozzle, which can get very messy.
Glycerine can be used as a lubricant, but can dry and turn sticky if the airbrush isn't used for any length of time. Iwata do a specialist lubricant, but I find I rarely need to use it.
If you haven't used your airbrush for some time, spray through a small amount of thinners (I use Vallejo for acrylics, Windsor and Newton for enamel's), or IPA before the first use, just to free up and 'wet' the internal surfaces.
Darren.