What do I do if a job opens up again that I interviewed for in the past?

They sent me a letter afterwards saying that they will keep my resume on record for 6 months. It is within that time frame and the same exact position is open again. What do I do? Just sit and wait to see if they contact me? Or is there some rule of thumb that I should call? I probably shouldn't send in a resume again right? I heard that it shows desperation if you sent a company more then one copy of your resume (like all at once), and I'm uming that goes for this situation as well since they still have mine on file... I am desperate though, just can't let them know that! :P