Any tricks for stirring rock hard Tahini?

Unless you want to stir it all day, scrape the whole can into a bowl and mix it with your electric hand mixer. It will make it creamy smooth.

If the jar is the same lid size as a peanut butter jar use this:
http://www.amazon.com/Witmer-Company-Peanut-Butter-Mixer/dp/B003IQH72M/ref=sr_1_2?ie=UTF8&qid=1380915077&sr=8-2&keywords=natural+peanut+butter+mixer

Another tip is when you first buy it store it upside down a while...and flip it every time you see it. It'll mix on its own after a week or so.

Check out the same question here
http://food52.com/hotline/21070-is-the-any-way-to-blend-a-jar-of-separated-tahini-without-making-a-mess-of-it

Or maybe dip the jar in some warm/hot water to loosen it up.

No tricks; a butter knife and elbow grease (and patience).

The only other thing I can think of is to scoop the whole thing out into a food processor and let it rip.