Exercise should be a regular part of your day, not something you just throw in where it happens to work out right then and there. It should be something you make time for like- meals, sleep, kids, work, school......set aside a set time for exercise!
Pencil it in on your calendar just like you would any other important appointment. You will find that exercising at the same time and on the same days each week will help you stay on track. It will become easier and it just becomes apart of your life. Consider scheduling your exercise like an appointment. EX: Monday, Wednesday, Friday and Saturday at 6AM you have an appointment at the gym. Usually we are not late to appointments, you don't miss them, and you don't schedule things in its place.
For myself, I find early morning 6AM workouts to be the best fit for me and my family. I am usually home before the kids wake up and my husband then goes to work. This way my workout is over and done and I don't have it lingering in the back of mind all day. I don't have to worry about sitters, daycare, husband running late, I get to avoid the evening gym crowd and it just sets a positive vibe for the remainder of my day.