Some way, somehow, that should be your ultimate aim, for your app to become something somebody else feels compelled to use because it not only has value but it may actually be fun. Wikipedia has an excellent page devoted to the definition of habit formation and the role of cues and triggers.
A habit is an automated response by an individual to cues or triggers and they are more likely to form if the target behaviour is consistently repeated in a stable context. This repetition should evolve an association between the task and its cues.  in his behavioural model for persuasive design insists that for an individual to enact a behaviour they need to have Motivation, the Ability to do the task and have aTrigger present. App-based triggers may be as simple as a reminder notice with a log-in link because “ we have not seen you in a while “. Offline, an external trigger for exercise may be as simple as making sure your running shoes are near your front door.
Fogg gives a well worked example of the Facebook re-involvement trigger. Certainly, those tasks that have an association with an event e.g. taking glucometer readings pre and or post meal are generally easier to remember and become routine than those needing to occur based on a specific time. In explaining the role of Motivators in his model, Fogg identifies three; Sensation (Pleasure vs Pain), Anticipation (Hope vs Fear) and Belonging (Social Acceptance vs Social Rejection).
The trick for designers is to understand that when individuals have high motivation then difficult task accomplishment they feel no real impediment to getting it done but when their motivation wanes, persuading the same people to complete the easiest of tasks becomes the focus. Your app must cater for both possibilities. When it comes to accommodating the varying abilities of users, designers must make the target behaviour as easy as possible to perform. If for example you want to talk to an exercise physiologist on an app you should be able to see the EP’s calendar and make a booking, receive a confirmation and on the day of the call select Consult-Talk-End Call with a single button push.
Of course, trying to design a web app that becomes compulsive enough to use regularly and loyally is no easy matter.
Google Play is awash with goal management and “habit apps” to guide you through improving your sleep patterns or to more effectively managing a healthy diet plan. Recently,  conducted an extensive examination of 115 of such apps in a bid to better understand how they go about the habit-forming process with their users. The authors believe thatreminders and trigger events influence habit formation. While reminders support repetition and help users remember to complete the task, they impede habit development. In contrast, reliance on trigger events to cue task completion increases the automaticity of the new behaviour. They criticise the apps they examined for not being sufficiently grounded in the habit literature. A consistent finding was that these apps do not actually support habit formation by helping users associate their new behaviour with a trigger event. They rely instead on extensive implementations of self-monitoring and tracking functionality which although they have been found support behaviour change,may not necessarily encourage new habit formation.
Interestingly,  discovered that although those apps with trigger-event features effectively supported automaticity of habit, as a cue mechanism they tended to be forgotten easily. On the other hand, stock-standard reminder functions encouraged adherence well but fell down on increasing automaticity. Out of this work, the authors devised a three-point guide for apps developers:
Support Trigger Events
Ask them to select trigger events. In dialog.fit we provide you with a simple step by step way to create, alter and share your exercise PLANS. Monitor their behaviour to see if the task is completed; we enable this through our TRACK and NOTIFY systems.
Use Reminders to Reinforce Implementation Intentions
This is interesting and we won’t know how valuable it is to you until we build it and try it with you. The thinking here is that we should send a reminder out to you before you start your exercise plan.
Don’t encourage features that create an over-reliance on the technology
This is an interesting recommendation. In effect, the authors’ concerns are that a total dependence on the technology may obfuscate the crucial associations that need to develop between contextual cues and tasks. Certainly when it comes to exercise as medicine all of our users should be aiming to be sufficiently autonomously motivated, socially supported and task capable enough to go and exercise regularly without external reminders.
 B. J. Fogg, “A behavior model for persuasive design,” in Proceedings of the 4th International Conference on Persuasive Technology - Persuasive ’09, Claremont, California, 2009, p. 1.
 K. Stawarz, A. L. Cox, and A. Blandford, “Beyond Self-Tracking and Reminders: Designing Smartphone Apps That Support Habit Formation,” in Proceedings of the 33rd Annual ACM Conference on Human Factors in Computing Systems - CHI ’15, Seoul, Republic of Korea, 2015, pp. 2653–2662.