Post-hypnotic suggestions are used all the time, in everyday life, for example, drivers see a red light and stop (usually!).
Post-hypnotic suggestions work by linking two events together like seeing a red light and stopping, hearing a police siren and feeling nervous, or if someone has a phobia of spiders they see a spider and panic.
Generally the stronger the emotion associated with the post-hypnotic suggestion the more powerful it is.
The more ‘normal’ it is the more likely it is to be accepted.
People can easily overrule a post-hypnotic suggestion, when they either don’t want to feel controlled or if the suggestion goes against their beliefs or values or just seems ridiculous.
The context of the suggestion has a large part to play.
For example, if a hypnotist gives the suggestion to go into a trance when they say sleep, this is likely to only work in the therapy room, not if they meet in the street. Or if a stage hypnotist suggests acting like a cow, when they hear a certain word, it will only work in the stage show where the person accepts it is for entertainment and is OK with the situation and willing to follow along for fun.
The suggestions are highly unlikely to be carried out if the subject is uncomfortable with the suggestion or if it was presented to occur in a different context (during a business meeting the next day, for example).
Mental rehearsal is a form of post-hypnotic suggestion. For example, you may have imagined things happening a certain way, in a certain situation, in the future. When that future situation becomes reality it triggers the rest of the suggestion. There is an increased likelihood of carrying out the patterned response, as a template has been created for it in the mind.
Sometimes mental-focus can be centered on just one part of an experience. For example, if someone wants more confidence to speak up in business meetings, it may be necessary to find a trigger at the meeting which will cause the suggestion to take effect. As the meetings may be in different locations the trigger needs to be something that will always be the same at each meeting.
It is often useful to focus on looking at their watch or phone, to get the feeling of confidence, or looking at their special pen. Perhaps the trigger is as simple as a pressing the index finger against the thumb, whatever trigger that has been linked to confidence in the subconscious.