This has already been posted couple of time already bust still posting this with little more details around using Impersonation SharePoint Designer.
Starting SharePoint Designer 2010 MS introduced this new step called “Impersonation Step” where any activity used inside this step will run under the workflow author (person who last edited the workflow in designer). Mind you this does not mean the person who started the workflow.
To use Impersonation step you simply click below your current step and from the ribbon click Impersonation step.
Once you add this step you will see a new step added to your workflow like shown below.