Maintaining a relationship is a long and difficult task. Both people have to be committed to the partnership. When your girlfriend becomes bored with you, your relationship will inevitably wane. Some warning signs are more serious than others. Your girlfriend may simply be asking you for change, or she might be finding change entirely outside your relationship. These signs are not absolute and are meant to be taken as possible calls for action.
Non-verbal signs are subtle yet alarming hints that your girlfriend is starting to lose interest in you. Public displays of affection, for instance, are a huge outward sign of a relationship. Conversely, if your girlfriend suddenly finds PDAs cumbersome or unnecessary, she doesn't feel the need to show that she's with you. Lack of eye contact is also a significant sign; it indicates that her mind is somewhere -- or with someone -- else. As subtle as these signs are, you should take note of them as soon as possible to avoid further issues down the road.
Communication is the lifeblood of any relationship -- lack of true communication spells impending disaster. If you find that she's consistently zoning out during a conversation or allows phone calls to interrupt your talks, these are signs that she's finding what you have to say less important. You should also look out for key words such as "not again" and "really?" when she talks to you. When she's using these words, she's trying to fish for something more exciting out of you. Take it as a sign to change it up.
If your girlfriend starts wanting distance more frequently, her boredom has become palpable. At this point, she may have given up hope on you and is looking for something different. A telltale sign is if she is more excited when you leave than when you're present. It's her non-offensive opportunity to hang out with her friends or find an out of your relationship. If she asks for a "break" for your relationship, this may mean that she wants to find a good way to end it. At this point, it may also be time for you to assess your relationship and, if necessary, find your own out.
Sex is infamous as a driving factor behind a relationship. Some people consider this as the last resort to bring back the sparks of excitement to a fading relationship. Conversely, if you're sex life is waning, it means that your girlfriend already considers your relationship as "fading". She may suddenly start acting out of character -- suddenly asking for a high level of physical affection or for the two of you to do things you haven't done before. She may bring this up at the oddest times as a joke, but this also means that it has crossed her mind. The motivations for this demand greatly varies; it is your responsibility to find out what her exact motivations are.