Well, is she flirting with the other guys back?
Decide if you really want to be with this girl: is she really worth it? Also, is she actually flirting? Some girls are just naturally friendly, and sometimes seem as though they're flirting when, in truth, they are completely dedicated to you. It may be that you are simply too possessive - your girlfriend should be able to talk with other guys in a friendly, nice way. After considering this, if you decide that she is flirting and not simply being friendly, and that it is a dealbreaker for you, tell her of your decision, and tell her why - perhaps it will force her to consider how her actions might be costing her good relationships.
Determine whether you are supplying her with enough attention. Maybe the reason she is seeking attention from other men is because she isn't getting enough attention from you. Try supporting her more and flirting with HER. If you find her flirting with one guy, you could approach her and hug her, kiss her on the cheek, or put an arm around her and give her a big smile, distracting her from other guys. If she is getting good attention, respect and attentiveness from you, she should not need to seek it elsewhere.
Talk to her. Politely ask her why she's flirting with other guys. Keyword: POLITELY, and sincere. Avoid becoming accusatory or hostile if possible, but let her know that you feel her flirting compromises your relationship with her. Take her aside, or wait for a moment away from other people at a later time to talk, and when you do, approach her with frankness and compassion, but with a firm resolve to put an end to the problem. Here are some examples of ways to honestly approach her about it without picking a fight
Emphasize the respect issue. you are not accusing her of being unfaithful or cheating. You are telling her that you feel disrespected when she behaves this way. Make sure she understands that, in your eyes, that's what it all comes down to - does she respect you enough to curb her impulse to flirt?
Ask her to put herself in your place. Don't start flirting with other girls to give her a taste of her own medicine - yet. Instead, ask her to imagine how she would feel if you seemed uninterested in her, but began hitting on all the single women at every party you went to. Get her to really think about it. If worst comes to worst, you could flirt with other girls, but it would be even better if you had a female friend or two that you could collaborate with - a setup. By getting a couple of girl friends to help you out by flirting with you, so that you can flirt back you would show her how this feels, and avoid misleading an innocent girl. By the time she sees you walk up in that "Joey" way, saying, "How you doin?" once or twice, she'll get the picture. If that doesn't stop her...
Don't become suspicious or accusatory; that could be a one-way ticket to splitsville.
Don't put up with it indefinitely. Do give her some time to make changes in her behavior, and to acclimate to a more reserved demeanor when you go out. But if she makes little or no progress toward stopping this behavior, you may not be able to salvage the relationship.