When the rate of blinking increases, it's a solid nonverbal signal that people are nervous.
It is well known that blink frequency increases when anxiety increases. Provided that the eyes are in good health and the air is relatively humidified, men blink about every 7-8 seconds and women blink about every 5-6 seconds. There is some variability to this, and as with all body language, it is important to establish a baseline - i.e., what is "their normal". What's much more important though is any change in the blink rate.
In this video clip of President Obama, notice his blinking considerably increase in the middle portions of this video. This clearly indicates he is nervous about poking fun at Mitt Romney. Let the games begin.
See also: Secret # 1149: Who was More Nervous in Last Night's Debate? Santorum, Romney, Gingrich or Paul?