Make a sum once per time, in the update function

I have this code, the commented part i want to do once per time, and because i need to put this code in the update function is constantly do the sum every frame, but i just need once per time when the player press the the arrowKeys. i Try to recreate this game: http://youtu.be/muJFwvf6gNE?t=1m44s especially the speedbar. Thanks and sorry if my english suck.

 using UnityEngine;
 using System.Collections;
 
 public class Player : MonoBehaviour {
 
     
     
     private bool avanzar = false;
     private float velocidadAvanza = 15f;
     private float sumaPower = 0.1f;

     void Start () {
     
     }
     
      void Update () {
         
         Avanzar();

     }
     
     void Avanzar()
     {
         if(Input.GetKeyDown(KeyCode.RightArrow))
         {
             if(avanzar==false)
             {
                 transform.Translate(Vector3.right * velocidadAvanza * Time.deltaTime);
                 avanzar = true;
                 PowerBar.restaPower += sumaPower; // <== Do this only Once
                 
             }
         }
         
         if(Input.GetKeyDown(KeyCode.LeftArrow))
         {
             if(avanzar==true)
             {
                 transform.Translate(Vector3.right * velocidadAvanza * Time.deltaTime);
                 avanzar = false;
                 PowerBar.restaPower += sumaPower; // <== Do this only Once
                 
             }
         }
     }
 }