Source: http://www.topendsports.com:

The measurement of resting heart rate (the number of heart beats per minute) should be taken after a few minutes upon waking whilst still lying in bed. Give your body some time to adjust to the change from sleeping before taking your pulse (2-5 minutes). If you are not able to take a measurement first thing in the morning, make sure you lie down for at least 10 minutes before taking a measurement. Taking a radial or carotid pulse measurement (at the wrist or neck) is usually the easiest method.

Get a baseline measure

You can monitor your resting heart rate over time, writing down the heart rate each morning for a few weeks. After a while you will get an idea of what your average resting heart rate is. Once a normal resting heart rate has been established, it becomes easy to determine your physiological state.

Changes in Resting Heart Rate

During a period of training, small changes in resting heart rate can reflect adaptation processes, or just a normal responses to the previous days training load. Resting heart rates can also be affected by ensuing illness, fatigue and overtraining. Also be aware that other factors such as smoking and caffeine, and some medications, can cause changes in resting heart rate. If your resting heart rate is 10 beats per minute or greater above normal then please let your coach know, and if it persists you may want to see your doctor.

What should it be?

Normal resting heart rates range anywhere from 40 beats per minute up to 100 beats per minute. Ideally you want to have a resting heart rate between 60-90 beats per minute. The average resting heart rate for a man is 70 beats per minute, and for a woman 75 beats per minute.

How Fit are you?

As you get fitter, your resting heart rate should decrease. This is due to the heart getting more efficient at pumping blood around the body, so at rest more blood can be pumped around with each beat, therefore less beats per minute are needed. See this resting heart rate chart which shows the expected heart rate for different ages and levels of fitness.

Another source: www.acefitness.org

The resting heart rate is most accurately assessed when measured for a full minute first thing in the morning before you get out of bed. Place your finger tips on the side of your neck (carotid pulse) or at your wrist on the thumb side (radial pulse). Avoid using too much pressure when using the carotid pulse to measure heart rate. Another source: http://www.the-fitness-motivator.com The best time to measure your resting heart rate is first thing in the morning…while still in bed. Take your forefinger and middle finger and place them on your wrist, neck or temple where your pulse can be felt the strongest. Looking at your wristwatch or a clock with a second hand, count the number of beats in a 15 or 30-second period. Multiply this number by 4 for 15 seconds or 2 for 30 seconds to get the number of beats per minute (bpm). This is your resting heart rate.