What advice would people give to someone who struggles with delaying gratification?

At its core, this is a question about habits, specifically, how to lose harmful habits and how to make beneficial ones. Requires focus, discipline and persistence. To do what? To invest the time and energy necessary to accurately diagnose the three essential parts of a habit, namely, it's cue, reward and routine. A few years back, Charles Duhigg came up with this comprehensive infographic to help deconstruct habits.

As the graphic explains, accurate diagnosis is necessary and requires time, energy and interest to experiment. Experimentation in turn is necessary to uncover the cue and reward system underlying a harmful habit.

A crucial idea is Keystone Habit, i.e., the power of some habits to trigger a chain reaction of other habits. An ecological concept that finds its place in anything from healthy microbiota to sociology, implicit in keystone is the notion that some habits are simply more powerful than others. Thus, the process of deconstructing habits involves as well identifying those that are keystone. Unsurprisingly, willpower or self-discipline is a core keystone habit. 

For e.g., some weight-loss studies showed that forcing oneself to form the new habit of keeping and logging entries in a food journal, i.e., a food log, turned out to have unexpected benefits. For many participants a once-weekly exercise even became daily and helped them notice previously undiscerned patterns in their behavior, plan healthier menus and make more conscious decisions about eating healthier food. Though Duhigg wrote an entire book on the Power of Habit, his New York Times article on the same topic is sufficient to get the idea: How Companies Learn Your Secrets

 

Obviously an obstacle in the quest to change habits, cognitive dissonance is another necessary factor to consider. It arises from different systems we have for assigning value. Thus, diagnosing the source of conflict between different valuation systems with respect to a harmful habit, say, habitual unhealthy snacking, is also necessary (see figures below from Rangel, Antonio, Colin Camerer, and P. Read Montague. "A framework for studying the neurobiology of value-based decision making." Nature Reviews Neuroscience 9.7 (2008): 545-556. http://worthylab.tamu.edu/Course...).

Other tips that could help include

• Starting with baby steps, i.e., scaling down the scale of change one seeks, something not so easy to do since the ego often demands or seeks a much more ambitious scope. However, though difficult, setting realistic goals is necessary.
• Not giving up immediately at the first sign of setback.
• Being patient with and forgiving of oneself.

Finally, any number of tips and tricks aren't going to do the trick by themselves. Habit is the result of many, largely unconscious, decision making processes. Intellectually deconstructing them can be relatively easy, even fun. Emotionally engaging with the process is the hard part. Unless engaged in the process both intellectually and emotionally, habit changing is likely to fail.

https://www.quora.com/What-advice-would-people-give-to-someone-who-struggles-with-delaying-gratification/answer/Tirumalai-Kamala