What's harder than talking yourself into making difficult but necessary changes in your life? Getting the people you love (or even the people you work with) to change.

At one time or another, most of us have been told that trying to change other people is a fool's errand. Nagging is just annoying, motivational speeches almost always fail to motivate, and constantly carping about the other person's deficiencies is a recipe for disharmony. Better to count your blessings and practice acceptance.

This is wise advice by and large, but what about the friend who actively asks you to help him start an exercise program or the colleague who wants you to push her to improve her productivity habits? What about the loved one who complains constantly about their ill health and poor diet? Do you just leave them to their fast food addiction and unhappiness?

Catch a motivational wave

Respecting others versus helping them can be a fine line to walk. Psychologist BJ Fogg can help. As director of the Persuasive Technology Lab at Stanford he studies how to motivate and influence others. In an interview with I Will Teach You to Be Rich author Ramit Sethi a few years back he provided useful insight into how to nudge others to change, without ruining your relationships.

"It may be that you have somebody that you're married to or someone in your family that you think needs to get more active or eat better or what have you. What you will probably be incredibly unsuccessful at doing is nagging them until they get it done. It doesn't work," Fogg insists. Motivational speeches and making them watch The Biggest Loser are also pretty sure to fail, he continues.

Instead he recommends, "waiting until something happens where they naturally get motivated." Fogg calls this a "motivation wave." When the other person's interest in changing peaks in this way -- say at the New Year or after tax season reveals terrible financial planning -- that's the time to strike, he continues.

"When that moment's right -- they're in a motivation wave -- then you help them take the steps they need to do to move forward," Fogg explains. "The motivation wave will go away. Motivation doesn't always stay high... so what you need to do when the motivation is high is get people to do hard things that then make future good behaviors easier to do."

Fogg offers the example of when he wanted to start drinking more tea. When he found himself fired up about this project, he went out and bought all the necessary supplies -- the tea, teapot, etc. -- and set them up. That way, when his motivation waned he had everything he needed right in front of him. Keeping to his new habit became  easy.

Switching to tea is obviously a small change, but the same principle could apply to something more difficult like getting fitter. When your better half is griping about getting into a swimsuit right before a family beach holiday, try to ride that motivation wave and get your partner to sign up for a starter package with a personal trainer or for a new activity, for example. Maybe spend an afternoon together cleaning the junk food out of the house and going shopping for healthier alternatives. That way when you're back to the usual daily grind and your loved one is feeling less excited about hitting the gym or eating kale, things will be set up so he or she needs less motivation to keep to the program.

Looking for other scientific insights on how to change your habits and increase motivation? Studies turn up interesting findings all the time. Or check out the complete interview with Fogg below for more insights.