From http://answers.unity3d.com/questions/403697/rts-healthbar-gui-box-follow-gameobject-with-persp.html

Hp_bar_unit

using UnityEngine;using System.Collections;  public class Attackable : MonoBehaviour {    public float maxHp;    public float currentHp;    public Camera     mainCamera;    public Texture2D  hpBarBG;    public Texture2D  hpBarCurrent;     private float offsetY;     //script will be atached to each GameObject that can suffer dmg.        void Start () {            //put it on top of the character        offsetY=gameObject.collider.bounds.size.y+5;        Debug.Log(offsetY);    }     void Update () {    }     void OnGUI(){            //the hp bar background location on screen        Rect rcBg = new Rect(0,0,60,14);            //the current HP bar         Rect rcCurrent = new Rect(0,0,((currentHp)/maxHp)*56,8);             //the important part : where this object is located in my screen        Vector3 point = mainCamera.WorldToScreenPoint(new Vector3(                                                                           transform.position.x,                                                                           transform.position.y+offsetY,                                                                           transform.position.z)                                                                          );             //had to add the substraction since my Y axis were revesed (            rcBg.y = Screen.height-point.y;               //do some ugly hardcoding so the bar is pretty and centered           rcBg.x = point.x-30;        rcCurrent.x=rcBg.x+2;        rcCurrent.y=rcBg.y+3;        GUI.DrawTexture(rcBg,hpBarBG);        GUI.DrawTexture(rcCurrent,hpBarCurrent);     } }