It's not the individual impact of slipping up that is a big deal. It's the cumulative impact of never getting back on track. (i.e. If you miss one workout, you won't suddenly be out of shape, but missing 3 weeks will make a difference.)
For that reason, it's critical to stick to your schedule, even if it's only in a very small way.
Find a way to stick to the schedule, no matter how small it is. Here's another example...
Let's say you woke up today with the intention of running 3 miles this afternoon. During the day, your schedule got crazy and time started to get away from you. When you look up at the clock, you only have 20 minutes to workout.
At this point, you have two options.
The first option is to say, "Twenty minutes isn't enough time to change my clothes, put on my running shoes, and workout." At this point, you'll probably convince yourself that your time is better spent getting caught up on emails or making a phone call you've been putting off ... or a thousand other alternatives.
This is what I would usually have done in the past - get pressed for time and come up with a good reason for not working out at all.
The second option is to reduce the scope, but stick to the schedule. You could tell yourself, "Twenty minutes isn't enough time to get changed and run 3 miles, but it is enough time to put on my shoes and run 5 sprints."
On a daily basis, the impact of doing five sprints isn't that significant, especially when you had planned to run 3 miles. But the cumulative impact of always staying on schedule is huge.
Furthermore, this strategy allows you to prove to yourself that you can get something done even when the situation isn't ideal. It's the cumulative impact of always sticking to your schedule that will carry you to long-term success. That's how little goals become lifetime habits.