A simple, and often overlooked, tool in increasing your strength is a notebook. This notebook can serve as your training diary/journal, in it you can write how your workout went with the difficulty and how you felt before, during and after. If you want to be really detailed you can also include your nutritional intake (the day of, and the days before the workout) and sleep quality in order to make evaluation easier.
The factors mentioned above can all be included in your self-evaluation. I'm sure many of you have had a particularly brilliant workout and wondered just why it happened as opposed to all those other workouts that have seemed like a mere grind. This is the way to figure it out. When keeping your diary be as detailed as you can, things to jot down would be : -
the workout itself, i.e the sets, reps and exercises
the rest inbetween
your sleep quality the night before
your nutritional intake, both the day of and the day before
how you felt before, during and after (physically and emotionally)
how busy your day has been
I'm sure that there are many more but even with these factors included in your evaluation you could figure out what it is that causes a great workout, or alternatively a poor one. Another obvious benefit of keeping a training journal is that you have a record and can see how far you've come, how much volume/intensity you have manipulated and how much progressive overload you have implemented.