When I first started as a fitness director at a commercial gym, this was the training log they used:

I created a more detailed training log that allowed members to track their aerobic activity as well:

I then came up with a log that was used strictly for strength training (click here for a high-res copy):

A training log is crucial for tracking purposes. In order to truly determine whether progress is being made, you must keep a detailed log of every training session. In addition to the date and start/end time of a workout, record the load used, number of repetitions performed, and even the time under tension (TUT) of every set. Special notes such as “feeling tired today”, “took 100 mg of caffeine pre-workout”, “right calf strain”, and so on can be useful for future reference. Highlight successful exercises and programs as well as any revisions you would make in the future. Without a log, how can you possibly keep track of all these details?