From Mens Health Magazine:
In order to tell if you're working out hard enough, you're going
to have to check with an expert: your heart. But the standard
220-minus-your-age equation isn't particularly accurate. A more
dependable method, and one that's correspondingly more
labor-intensive, is measuring your heart-rate reserve. This
formula includes your resting heart rate (RHR), a key indicator
of your fitness level. To measure your RHR, take your pulse as
soon as you wake up in the morning. (You should probably measure
your RHR three consecutive mornings and take the average. A lot
of factors can temporarily elevate it.)

Let's say you're a reasonably fit 35-year-old guy with an RHR of
80. To calculate your heart-rate reserve, subtract your age from
your predicted maximum heart rate (220 minus 35, or 185).
Subtract your RHR from that figure (185 minus 80, or 105).
Multiply that number by the top intensity at which you might
train - 85 percent - and you get 89. Add your resting heart rate
and you get 169 beats per minute at 85 percent of maximum
intensity. Standard heart-rate calculations would have told you
85 percent of maximum effort was 157 beats per minute, or 12
fewer.