Extend an invitation to your best friend to meet in a place that is both private and quiet. Making a heartfelt apology is best accomplished in an environment where you both can listen as well as communicate. It's also important to meet in person, rather than apologizing via text, email or voicemail, because what you communicate nonverbally can be just as important as what you say. Agree on a location, but avoid making plans to have dinner or drinks together. Until you have made your apology, you won't know if your discussion will continue. Dinner, in this case, becomes an awkward obligation, and drinks can impair your judgment.

Provide a statement of regret that includes the specific behavior for which you are apologizing. This statement, explains psychotherapist Beverly Engel in the University of Massachusetts Amherst's online publication "How to Give a Meaningful Apology," should be given with a sense of empathy for your friend. Keep in mind that it's nearly impossible to fake empathy because you also express it nonverbally through open body language, good eye contact, attentive listening and smiling when appropriate.

Acknowledge the hurt and other feelings expressed by your best friend. Put yourself in your friend's place and imagine how you would feel if she had been the one kissing your boyfriend. Allow your friend time to express how she feels about what happened and take her at her word, acknowledging that "I understand that you are feeling betrayed, and I can see why you wouldn't want to talk to me." It's also helpful to include an opinion about your behavior, such as "I didn't think about your feelings."

Express your willingness to take action to make things as right as possible with your friend, and assure her that it won't happen again. Engel explains that this approach includes a suggestion as to how to remedy the situation. In this case, a viable remedy may be to have a discussion with your best friend and her boyfriend, explaining that you will not kiss him again. You also can let your best friend know that you will not spend time alone with her boyfriend, so that you can begin to rebuild her trust. Also, ask your best friend what she feels you can do to help repair the friendship.