Ditto on water always being enough for me. I've also heard to use Visine. You could always see about using MAC's water-based or eyeliner Mixing Medium (available at Pro stores).
The eyeliner one is described as:
Quote:
A colorless, translucent mixing medium that can be mixed with pigments and glitter to create unique customized eyeliner shades. Contains Vitamins A, C and E to buffer the delicate skin around the eye area from environmental aggression. Non-irritating and suitable for all skin types. Water-based is described as: Quote:
Formula to dilute and mix with M·A·C powder or liquid makeup. Used to create effects from sheer to opaque. Perfect to decorate the face or body. Created for those who like to design their own shades and colours. Mix with pigments and powders to create anything from full, opaque coverage to sheer. Useful for adhering M·A·C Glitter to the skin.