Well, it kind of depends on what I\'m doing at the moment as to what additives I use.
If it\'s just general thinning, I usually use either distilled water or Vallejo Thinner. The thinner is actually kind of nice, as it (supposedly) won\'t weaken the bonding agents, unlike water. I use distilled water because it doesn\'t seem to make the paints go as chalky (probably due to high mineral content in my tap or possibly Chlorine).
If I\'m doing washes/glazes, then I use Vallejo Glaze Medium, and possibly some \"wet water\" (a mix of distilled water and Future Floor Finish or other flow improver). If it starts to look shiny, then I\'ll sometimes add some Matte Medium, too. I find this really helps reduce \"tide marks\" better than using water alone.