There is nothing like writing it down on paper! For years I have written what I do at the gym on my calendar. According to the period, I have it that I must work out a certain amount of days per month, doing certain body parts or exercises a certain amount of time. Writing it down makes it real and helps me to keep track.
Other tricks you may want to try is writing down what you eat each day, morning to night, if you are on a diet. You can keep track of vitamins, nutrients, and calories.
It is also smart to write down your periodic exercise programs. You can do something like this:
Day: Monday Tuesday Wednesday Thursday Friday Saturday Sunday
and so on
Then you can check which day of the week you worked on what muscle.
Alternatively, you can write the days of the week vertically down the right of the page, and in the squares fill out the exercise or machine, and sets and repetitions for each muscle group and day.