"My girlfriend needs me. GYM TIME!"
Nothing says amazing boyfriend like never being around when you need him. That time when your dog died and your heart was breaking; "Babe, I need you here... I am in so much pain." You initial reaction will be to get in your car and be there for her. But DON'T! Don't be that bad boyfriend that no one wants! Chicks don't dig that.
The moment you feel the urge, start thinking up some average excuse that may or may not be believable. Better yet, don't even offer an excuse or reason why you can't be there. Keep the conversation short and basically sum up the necessities: "I'm sorry you are feeling bad, feel better soon. Check you later!" See, the trick is to say the right things and not appear to be horrible when in actual fact you are ducking out of the ENTIRE situation. Genius.
So next time you want to be there for the one you care about, remember that will only make you look loving and caring. And who could possibly want that?