Does it really only take 21 days to create a habit? This hopeful concept has sold a lot of books. But if it were true, we’d all be slim, avid exercisers, nonsmokers and achieving huge work successes by next month. Instead, we muddle through on a much rockier road to behavior change.

Making a big change is rarely a matter of modifying one habit. Instead, it’s about changing many habits. Take weight loss, for example. Losing weight might require bringing exercise clothes to work, going to the gym four times a week, regular grocery shopping, meal planning, cooking, eating only at certain restaurants, having meals at certain times, walking places you used to drive, drinking seltzer instead of soda and the list goes on. Reaching your weight loss goal might require adopting 10, 20 or more new habits. For each person, the set of habits is different depending on current habits and how much weight he or she wants to lose. The first question, then, is not how to change a habit, but rather, which habits to change.

As a clinical psychologist, I have worked with hundreds of people on weight loss. One of the biggest mistakes I see people make is investing energy in the wrong habits, meaning those habits that aren’t most likely to help them achieve their goal. The right habit should meet two criteria: First, it should address a major impediment to your progress; second, it should have a measurable return on investment, or ROI. Let me explain with an example.

Jeanine – a fictional patient I’ve created based on the experiences of actual patients – wants to lose 40 pounds and is working on a few healthy habits she believes will help her achieve this goal. First, she says she wants to improve her diet by eating more fruit and vegetables. She plans to put berries on her cereal each morning, bring an apple to work for a snack and add more veggies to her dinner recipes. She implements these habits but finds herself frustrated because the scale hasn’t budged.

Jeanine doesn’t realize that these healthy habits aren’t tied to any of her major weight-loss impediments. A look at her diet reveals that heavy snacking in the evenings and overeating at restaurants on the weekends are the two biggest sources of extra calories in her diet. While it's great that she's increased her fruit and veggie intake, she hasn't reengineered the unhealthy habits that caused her to be overweight.

The second habit Jeanine chose to focus on is increasing her physical activity by parking farther away and taking the stairs at work. These healthy habits fail the ROI test, contributing nominally to her energy expenditure by burning only about 40 extra calories per day. One might say focusing on these small steps could motivate a person to engage in additional activity, but more often it ends up inhibiting additional efforts to be more active. The problem with focusing on the wrong habits is that it produces a false sense of making progress and eventually leads to frustration and failure.

Jeanine is making progress on some healthy habits, but these habits aren't pushing her closer to her weight-loss goal. Eventually this erodes her motivation. I call habits that aren’t connected enough to the goal disconnected habits. Investing time in disconnected habits is not only exhausting, but it eventually leads to failure. We only have so much mental energy for behavior change, so it’s best to invest it wisely. Laser-focusing on habits that will deliver you to your goal is most likely to produce big dividends.

To identify connected habits, the first step is to identify the major impediments to your progress. It may be, as in Jeanine’s case, that nighttime snacking, weekend eating and hours spent watching TV in the evening are the biggest obstacles to weight loss. In such a case, I would brainstorm a big list of healthy habits that could help a patient chip away at these impediments, and then suggest selecting two habits to focus on first. That might be going to the gym at least three nights a week not only to get in some moderate intensity exercise, but also to occupy at least one hour in the evening that might otherwise be spent snacking. A second change could be to restrict evening snacks to fruit and veggies. As in the scenario I’ve outlined, this habit would tie in the desire to eat more fruits and vegetables, but in a way that would more effectively reengineer an unhealthy habit – in particular, nighttime snacking. Ultimately, the point is to make small changes tailored to your specific unhealthy habits, such that you see results – like weight loss – and create momentum to make more healthy changes in the future.