vector gPosSlide = < 0.5, 0.0,0.0 >; //-- 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() );
    }
}