I've written at length about implementation plans in my book “Planning to Remember: How to Remember What You're Doing and What You Plan to Do”. Essentially, they're intentions you make in which you explicitly tie together your intended action with a specific situational cue (such as seeing a post box).
A new study looked at the benefits of using an implementation intention for those with low working memory capacity.
The study involved 100 college students, of whom half were instructed to form an implementation intention in the event-based prospective memory task. The task was in the context of a lexical decision task in which the student had to press a different key depending on whether a word or a pseudo-word was presented, and to press the spacebar when a waiting message appeared between each trial. However (and this is the prospective element), if they saw one of four cue words, they were to stop doing the lexical task and say aloud both the cue word and its associated target word. They were then given the four word pairs to learn.
After they had mastered the word pairs, students in the implementation intention group were also given various sentences to say aloud, of the form: “When I see the word _______ (hotel, eraser, thread, credit) while making a word decision, I will stop doing the lexical decision task and call out _____-______ (hotel-glass, eraser-pencil, thread-book, credit-card) to the experimenter during the waiting message.” They said each sentence (relating to each word pair) twice.
Both groups were given a 5-minute survey to fill out before beginning the trials. At the end of the trials, their working memory was assessed using both the Operation Span task and the Reading Span task.
Overall, as expected, the implementation intention group performed significantly better on the prospective memory task. Unlike other research, there was no significant effect of working memory capacity on prospective memory performance. But this is because other studies haven't used implementation intentions — among those who made no such implement plans, low working memory capacity did indeed negatively affect prospective memory performance. However, those with low working memory capacity did just as well as those with high WMC when they formed implementation intentions (in fact, they did slightly better).
The most probable benefit of the strategy is that it heightened sensitivity to the event cues, something which is of particular value to those with low working memory capacity, who by definition have poorer attentional control.
It should be noted that this was an attentionally demanding task — there is some evidence that working memory ability only relates to prospective memory ability when the prospective memory task requires a high amount of attentional demand. But what constitutes “attentionally demanding” varies depending on the individual.
Perhaps this bears on evidence suggesting that a U-shaped function might apply, with a certain level of cognitive ability needed to benefit from implementation intentions, while those above a certain level find them unnecessary. But again, this depends on how attentionally demanding the task is. We can all benefit from forming implementation intentions in very challenging situations. It should also be remembered that WMC is affected not only more permanently by age, but also more temporarily by stress, anxiety, and distraction.
Of course, this experiment framed the situation in a very short-term way, with the intentions only needing to be remembered for about 15 minutes. A more naturalistic study is needed to confirm the results.
 . The Compensatory Role of Implementation Intentions for Young Adults with Low Working Memory Capacity. Applied Cognitive Psychology [Internet]. 2015 :n/a - n/a. Available from: http://onlinelibrary.wiley.com/doi/10.1002/acp.3151/abstract