How To Move An Element (with arrow keys) Consistently???

How To Move An Element (with arrow keys) Consistently???

I have a DIV set up to move whenever I press an Arrow Key. It moves fine with one push, by holding the arrows is a little messed up. When I hold an Arrow Keys, the DIV moves in one increment THEN it continues to move, as I hold the Arrow Key.

How do I get rid of the first increment? I want the DIV to move consistently when I hold an Arrow Key. Here is my JavaScript code:

$(document).keydown(function(e){
    switch (e.which){
    case 37:    //left arrow key
        $(".ball").finish().animate({
            left: "-=20"
        });
        break;
    case 38:    //up arrow key
        $(".ball").finish().animate({
            top: "-=20"
        });
        break;
    case 39:    //right arrow key
        $(".ball").finish().animate({
            left: "+=20"
        });
        break;
    case 40:    //bottom arrow key
        $(".ball").finish().animate({
            top: "+=20"
        });
        break;
    }
});