2d - How to correctly implement player acceleration caused by player input

In my game I implement movement in the Player class like so:

// called from Player's update() method, which is called from the gameloop.
void move(){
    velocity.x += acceleration.x;
    velocity.y += acceleration.y;
    position.x += velocity.x;

    position.y += velocity.y;
}

Keyboard input affects the acceleration directly, for example:

// in Player, called from main gameloop.
void update(){
    if(playerInput.rightPressed()) (acceleration.x += 2);
    // .. etc
    move();
}

I think so far this is standard (if not, please say so).

Now my question:

During movement invoked by player input, I want the acceleration to be constant. So when the player presses the right arrow key, the acceleration.x will always be 2 - and not 2, and then 4, and then 6, etc.

I could easily acheive this be having if(playerInput.rightPressed())(acceleration.x = 2), but then other forces acting on the entity will be ignored.

What is the standard way to implement this?