I see that a number of people have already given good answers to this. My approach is to look for at least 1.5 cents per mile value on an airline where Im not working towards status or 2 cents on airlines where I am working on getting status. Here's a post I wrote on this:
http://boardingarea.com/blogs/frequentmiler/2011/12/23/when-is-an-award-flight-a-good-deal/