CRM 2011: Updating record the plugin was fired by / from does not work.

1- Register in Async-post-update with the check of

if (context.Depth > 1){ return; }  //To Avoid Multiple Trigger Of Same Update Plugin.

2- Register in Sync-post-update:
Try Creating a new Object for Updating the record Like

  Entity test=new Entity("contact");
  test.Id=((Entity)context.InputParameter["Target"]).Id;
  test.Attributes["firstname"]="ABC";
  service.Update(test);