Depends if you're airbrushing or just brushing. For just brushing, I jump around. Sometimes water cut with just a bit of dishwasher fluid. Sometimes Vallejo's thinner. Sometimes a few drops of Future.
For airbrushing, I had nothing but bad experiences until I tried thinning with Future. About 1:1 ratio, roughly. I tried water, their thinner, Windex, etc, and each time found a very fine line between the paint being too thick, or suddenly taking on water-like properties. With Future, it just goes on, and it dries smooth, solid, and shiny.