Understand the role proximity plays within your crush. Often you're more likely to develop feelings for people who you're physically around for a long period of time. If this is the case with your boss, it may suggest that your crush simply reflects how much time you spend together.
Appreciate that workplace crushes are normal. You shouldn't feel a sense of embarrassment or shame for experiencing a crush on your boss; this occurrence is common and a part of life. By seeing your crush in this context, it may help put it into context and minimize its strength.
Rationalize the situation. If you want to get over the crush, it's usually because the feelings are unconstructive and not desirable; for example, you may already have a long-term partner who you care for a great deal. Picture what the situation would be like if you and your boss actually formed a relationship, and consider how that may create negative or impractical results.
Create some distance. If you and your boss routinely communicate informally or outside of work, it may be practical to reduce the amount of non-business contact which you have between one another.
Meet new people. The simplest way to get over one crush is to develop a new one; to this end, make an effort to routinely meet new people within new environments. This can help take your mind off your boss.