The Rate of Perceived Exertion (RPE) scale is commonly used for determining exercise intensity.
I use it because it’s simple.  No fancy calculations, no maximum heart rate testing.  Just listening to your body.
Rate of Perceived Exertion (RPE)
Courtesy of www.exercise.about.com
Level 1: I’m watching TV and eating – bons bons
Level 2: I’m comfortable and could maintain this pace all day long
Level 3: I’m still comfortable, but am breathing a bit harder
Level 4: I’m sweating a little, but feel good and can carry on a conversation effortlessly
Level 5: I’m just above comfortable, am sweating more and can still talk easily
Level 6: I can still talk, but am slightly breathless
Level 7: I can still talk, but I don’t really want to. I’m sweating like a pig
Level 8: I can grunt in response to your questions and can only keep this pace for a short
time period
Level 9: I am probably going to die
Level 10: I am dead