2d - How do I implement deceleration?

I say "deceleration" because I'm not using acceleration at the moment; what I mean is moving the velocity back towards zero, eventually stopping.

I'm new to vectors and not so great with physics and such. How is "deceleration" normally handled?

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What I have now works, but it seems kind of hackish.

update:function(Game, t, dt) {
    var speed = Game.Input.isKeyDown('shift') ? 8 : 4;


    if (Game.Input.isKeyDown('a')) {
        this.velocity.i -= speed;
    }
    else if (Game.Input.isKeyDown('d')) {
        this.velocity.i += speed;
    }
    else {
        if (Math.abs(this.velocity.i) > 3) {
            this.velocity.i += (this.velocity.i > 0) ? -speed : speed;

        }
        else {
            this.velocity.i = 0;
        }
    }

    if (Game.Input.isKeyDown('w')) {
        this.velocity.j -= speed;
    }
    else if (Game.Input.isKeyDown('s')) {

        this.velocity.j += speed;
    }
    else {
        if (Math.abs(this.velocity.j) > 3) {
            this.velocity.j += (this.velocity.j > 0) ? -speed : speed;
        }
        else {
            this.velocity.j = 0;
        }
    }


    this.updateVectors(dt);
}

I used 3 because anything lower exhibits weird behaviour, I'm guessing if I raised the speed then it would need to be changed.

Answer

Something as simple as

this.velocity.i *= 0.9;

works nicely.