There are many positive and beneficial things that we know we should do, but we may feel that continuing to do them will be too difficult. "How can I keep being this way all the time?" we ask ourselves.
Telling ourselves, "Just once," can help us start. After we do something positive once, we begin to build up momentum; it becomes easier than we had assumed to keep doing the positive action more frequently.
Doing a worthwhile action "just once" doesn't seem so daunting. The next time it comes up, say you'll do it "just once" again. Even if it truly is difficult, since we only have to motivate ourselves "just once," right now, we can build up our level of inspiration and motivation.
Think of something that you would want to do but also consider difficult to repeat. At the next opportunity, start by taking action "just once." If you're having difficulty thinking of something to do "just once," keep thinking until you find an area to apply the power of "Just once."
(from Rabbi Zelig Pliskin's book: "Conversations With Yourself", p.148) [Artscroll.com])