Principiis obsta, a Latin adage. Roughly translates "resist (the) beginnings".
I have learned this adage way back when I was studying for the Catholic priesthood, in the context of moral theology. The idea, in that context, was the prevention of sin. Essentially the point was that if you resist the moment a temptation starts arising, you are less likely to commit the sin than if you dwell on it, think of it, consider what-if scenarios, and things like that.
Though I have long since converted to a different viewpoint, and the idea of sin does not even exist in my current spiritual path, I still find the idea of principiis obsta quite useful and practical.
Consider, for example, that someone is asking you to do something you don't want to do. Now, suppose you tell him that you'd much rather not (e.g., volunteer for a group while you are busy volunteering for other groups), and he says, well, just do it once, please, please, please. And you do it.
When he asks you for the second time, it gets harder to turn him down. And after you've been doing it regularly, it becomes virtually impossible to say no.
Had you said right at the beginning, "sorry, no" (instead of "I'd rather not"), that would be it.
Another example may be when the company you work for tries to get you to do some extra work with no extra pay. If you do it, they'll ask you for more and more. If you turn them down politely right at the beginning, they won't keep adding more work.
Or, suppose you start an Internet discussion forum and lay out basic rules but do not enforce them from the start, it becomes very hard to say several months later, "OK, folks, start following the rules."
Or, if your local government starts discussing a proposal for a regulation you do not like, and you object as soon as the city council starts discussing it, you have a good chance of tabling it. But if you wait for it to pass and only then object, it is much harder to have it overturned.
A similar idea is the English adage, an ounce of prevention is better than a pound of cure.