I was just impatient since my chiller is very inefficient in the summer.
Here's what I've done in the past in the summer - freeze 4 or 5 (20oz ) water bottles, cool with your chiller until it cools to ~ its limit (ground water temp), remove the chiller, and add the sanitized frozen bottles. You'll need to stir to keep a steady temp reading. It'll get you a few extra degrees of cooling. I've cooled to 60 or 62F this way.
EDIT - You need to be sure to remove the chiller when it hits its limit though, because at that point the steady temp going through the coils is actually resisting/slowing down your cooling the wort any further.