1. Age-predicted Target Heart Rate
220-age=HRmax (maximal heart rate)
Workout range: 55 - 90% HRmax
Example: Age 42
220-42 = 178
178 x .55 = 98
178 x .90 = 160
Workout range: 98-160
Wearing a heart rate monitor makes this much easier than stopping to check your heart rate.
Also, this is only an estimate. Everyone is different, so this isn't exact. For instance, I am 52 years old. My age-predicted maximum heart rate is 168 with my workout range 100 - 151. Yesterday I went running and my average HR was 155 and my maximum was 170 after running up a hill. The true way to find out your maximum HR is a Maximal Exercise Test (more commonly known as a stress test). For most healthy people, this isn't an option, as they are expensive and usually only done if you have symptoms of heart problems.
2. Rating of Perceived Exertion (6-20 scale)
6 No exertion at all
8 Extremely light
10 Very light
13 Somewhat hard
17 Very hard
19 Extremely hard
20 Maximal exertion
The goal with RPE is to work at a level that feels "somewhat hard" to "hard", or at a level of 13-16. The advantage to this method is that you don't have to keep checking your heart rate. You just go by how you feel.
3. Talk Method
I often tell my patients to work at a level where they can talk "haltingly". If they can carry on a conversation as if they are standing still, they need to work a little harder. On the other hand, if they are so out of breath that they can't talk at all, they need to slow down. (This rarely happens with my patients. The majority of them come to socialize, which is important, too.)
Wearing a heart rate monitor is a fun way to see how hard you are working. Polar is probably the most popular brand. Depending on the model, you can monitor your HR (with a readout at the end of your maximum HR and average HR), workout time, calories burned, and much more.