Fearful people don't like to sit around waiting for habituation to occur. Rather, their instinct is to run away; to escape the feelings they experience. Running away prevents habituation from occurring, however, and so is counterproductive to the exposure process. For this reason, efforts to prevent the fearful person's escape response from succeeding are typically useful for helping them get to the point where fear-habituation can kick in.
Caution! A careful balance must be struck here! No one should be prevented from escaping a fear when that prevention is against their will. Instead, they need to choose to be prevented from escaping. Forcing someone (even yourself) to remain in a fear-inducing situation when that situation becomes intolerable may make problems worse rather than help them to relax. Response prevention should be chosen; not forced.
There are a variety of ways to escape a feared situation, some obvious and some subtle. An example will help illustrate how a variety of different escape methods can be prevented. Some people are socially phobic, or afraid. They may be afraid to speak in public, or to ask someone they are attracted to out on a date. Socially phobic people often fear terrible consequences will befall them if they do speak in public or ask someone out on a date. They will mess up, they think, will be humiliated, and will be revealed as a failure. Generally, these fears are exaggerated; no one would really think that badly of them, but nevertheless, this is what they think will occur.
A date-phobic person might help himself to get over his fear by deliberately placing himself into situations where he needs to ask someone else out. Though he may know that he needs to do this, the fear he feels when he actually tries to pick up the telephone to call is massive, and his first impulse is to put the phone back down. If he gets to the point where the phone is ringing, he may experience an even more massive urge to immediately hang up before anyone answers. These urges to not call, or to hang up before a connection is made are obvious sort of escape techniques. They can be overcome by simply choosing to persevere despite the fear, or by recruiting a friend or family member (or therapist) to function as a cheering section, urging you forward and preventing your escape.
Once the phone connection is made, there are a number of other more subtle escapes that may be attempted. Our fearful guy may convince himself that the person at the other end of the line would never date him in a million years, and, working off that (very possibly false) assumption, not ask for a date at all. He might ask for a date, but do so in a very hesitant and self-defeating way, "I don't want to bother you....I suppose you're busy....You probably have better things to do....", which communicates his fear rather than his desire. If he gets a date, he may fail to kiss his partner good night, or otherwise subtlety move the relationship in a friendly direction rather than a romantic one. These subtle sorts of escapes are much harder to prevent because they are much harder to spot occurring
Your best chance at catching yourself attempting a subtle escape of this sort is to self-monitor in an informal way; to make a record or keep a journal of your experience, (what you thought as well as what you did) and to review that record or journal regularly so as to become aware of your tendencies The more aware you are of your subtle escape attempts, the easier it will be to avoid making them.