Measuring Unstable Laser Power

What is the best way to measure the power of a laser that is, unfortunately, not stable, where the power is drifting upwards or downwards. I am making a measurement taking readings by hand and logging to computer?
 
The best way to measure in such a condition is to take statistics of a number of readings. In order for the sample to be truly random, you should a given intervals look at the reading and take it down as seen with no attempt to wait for it to stabilize or reach a "better" value.