Habits are reinforced and learned through repetition and memory.
This week, sit down and write out what you want to accomplish each day. Maybe one day is a long run, another is 45 minutes of yoga and the next is a full hour of weights. Or start with one day of an alternating walk/run for 40 minutes, another day is yoga and another is 45 minutes on the elliptical. If your body isn't used to exercise, be sure to space out the harder workouts with less impact exercises. That doesn't mean easy, just use different muscles on different days to allow recovery.
Always keep in mind, too, that unplanned things can happen. So if something comes up where you can't make it to the gym that day, switch the workout with one from another day that you do at home. Or maybe you wake up and it's snowing so going for a run won't work. Don't take that as an excuse to not workout. Simply look at your written plan, and mix things up a bit.
Writing things down makes you accountable, and even better...crossing things off a list feels great! So plan away my friends!