To gives eyes that bright sparkle of sentience you can use a drop of a gloss sealer to set off the flat, non-gloss paints often used on eyes.

Also of use is Aleene's "Satin Varnish". This technique can be used to gives mouths and tongues slobber, shiny leathers, goggles, and belt buckles.

Jeremy Beckett suggests mixing some acrylic iridescent medium (a mother-of-pearl mix) to attain the same gleam. This is also particularly good for creating polished leathers and helmets.