It's not just a matter of guy using girl or woman using man. It's just a matter of humans using other humans. I've been used a few times before. There are people out there who will mistake kindness for weakness and try to prey on that "flaw". Sometimes it can be prevented, but there are times you feel you're being used, but don't know how or why. I've been there quite a few times.
The most you can do is try to find a way to change your situation. Y'see, I believe that the reasoning behind using other people is because deep down inside, that person doesn't want to help his/herself. So, they convince others to "help" them. It's worse when it's a relationship matter. I know a guy who used his girlfriend solely for her car. He didn't love her--hell, he didn't like her--yet, he would manipulate this woman, make her feel like she's wanted and he would sweet talk her in to doin' damn near ANYTHING; the car being his high priority.
It wasn't until they got in an accident that he realized he should focus on them and not on material possesions. I'd change too, if I flew through a windshield.