For example: I'm tired and don't feel like doing my workout. I ask myself - will I regret doing my workout? Never! Will I regret not doing it? Absolutely! So there I have it, clear cut answer and motivation to do my workout.
Another example: I love french fries, and out doing errands I happen to drive by a fast food restaurant. I ask myself - will I regret buying those fries? Um, yeah. Will I regret not buying them? Never! Another decision made, no fries for me.
Don't let regret run your life. Let not having any regrets run it!