Practice open and honest communication within the relationship. Encourage her to share her thoughts and feelings with you and, in return, share your thoughts and feelings with her. Poor communication is among the leading causes of failed relationships. If your girl does not feel as though she can express her feelings, thoughts, concerns, wants or needs without you getting annoyed with her, she could end the relationship out of frustration.
Respect her individuality. A girl who feels that her boyfriend allows her to make her own decisions is more likely to remain in a relationship than a girl who feels as though her boyfriend is always trying to change who she is. Avoid comparing her to other girls, and just accept that she is her own, unique person. Encourage her to make the best decision for herself in every situation.
Surprise her with romantic gestures from time to time. Chances are that you performed romantic gestures to capture her attention early on in the relationship, so continue to do so now. The key is to avoid overdoing it. If you're playing the role of the gentlemanly knight in shining armor every night, she may get bored. However, if you surprise her with dinner or a warm bubble bath every now and then, you can ignite a new spark in your relationship every time.
Exercise a reasonable amount of understanding. Recognize that, like everyone else, your girlfriend is human and that she will sometimes do things that get on your nerves. Try to understand where she is coming from in these instances before instigating an argument. When arguments do arise, remember Step 1. Effective communication is especially important when tensions are high, so respect and acknowledge her perspective during disagreements.
Focus on the positive. In "Introduction to Learning and Behavior," Russell A. Powell, Diane G. Symbaluk and P. Lynne Honey explain that, aside from effective communication, one of the key factors in many long-lasting relationships is a tendency to emphasize the positive. Instead of constantly pointing out your girl's flaws, remind her of the things that you love about her.
Love her and let her know it. Women, and people in general, have an innate desire to be loved. Tell her you love her often and, moreover, show her whenever you get the chance. There are many ways of showing her your love. You may also take care of a chore she has to do on a day she isn't feeling well or surprise her with a relaxing massage, for starters.