  Some fluids exhibit anomalously low friction when flowing against a certain
solid wall. To recover the viscosity of a bulk fluid, slip at the wall is
usually postulated. On a macroscopic level, a large slip length can be
explained as a formation of a film of gas or phase-separated `lubricant' with
lower viscosity between the fluid and the solid wall. Here we justify such an
assumption in terms of a prewetting transition. In our model the thin-thick
film transition together with the viscosity contrast gives rise to large
boundary slip. The calculated value of the slip length has a jump at the
prewetting transition temperature which depends on the strength of the
fluid-surface interaction (contact angle). Furthermore, the temperature
dependence of the slip length is non-monotonous.
