vector gPosSlide = < 0.0, 1.0, 0.0 >; //-- the offset amount to move

default{
    touch_start( integer vIntNull ){
        llSetPos( llGetLocalPos() + gPosSlide * llGetLocalRot() );
        gPosSlide = -gPosSlide;
        //-- or replace the previous 2 lines with...
        // llSetPos( llGetLocalPos() + (gPosSlide = -gPosSlide) * llGetLocalRot() );
    }
}