Gradual stop for scroller

programming flash
1

hey I have a as2 scroller button that drags ‘clip’ MC:


on(press){
this.onEnterFrame=function(){
speed=20;
llimit=-400;//right limit
if(clip._x>llimit){
clip._x-=speed;
}}
}
on(release,releaseOutside){
delete onEnterFrame;
}

how do I make the speed gradually (say 1 second after release to) halt?

Thanks,
Andy