Coat strings with a thin layer of string lubricant or a water-displacing spray. Lubricated strings slide more easily under your fingers and generate less noise.
Wash your hands before you play the guitar. Dirty hands create far more squeaking than clean ones.
Switch to nylon strings. Nylon strings do not squeak but they sound distinctly different from metal strings. You will have to try both to determine which works best for you.
Replace traditional nickel-wound strings with coated strings. These strings are coated with a polymer that helps limit squeak while maintaining the traditional sound of metal strings.
Replace round-wound strings with flat-wound strings. Flat-wound strings are more expensive than their counterparts but they minimize squeak.