Generally, I have problems sticking to any exercise plan, so here are a couple ways I handled it:

1. A while back I had a bit of spare money so I paid for a couple months with a personal trainer doing strength training. This was great because I learned "the right way" to use exercise machines and lift weights and combine exercises. Since I was paying, I made myself show up. This was a bit expensive though, so....

2. I meet up with a friend at the community rec center gym and use the exercise machines there. I make myself show up, since I committed to meet my friend.

3. Another thing I do is martial arts classes. If you want to get good at a martial art, you should practice at least 3 times a week, preferably more. But I don't do it that often, my motivation is that I make myself show up for the exercise because it's fun.

At the moment I go to the gym 3 times a week for 40 min workouts, and do a martial arts class once a week for a 2 hour session. During busy periods this can go down to just 2 times a week at the gym and no martial arts classes.