Point being, if she mentions some guy or flirts with another guy in front of you and you immediately go and game a girl, it makes you look try-hard. Plus, girls aren't so stupid as to not see the action-reaction connection. So just… do nothing. As mentioned above, if she talks about some dude and you can't constructively respond, say nothing and/or start a new thread of discussion. Or you can say "D'you… do you think I'd like him? Maybe you could… invite him over one evening. Just for dinner, of course… grin."
Another issue is that of… it just shouldn't get to you. OK, that is random alpha thingy advice, as any human being will feel SOMETHING, but still. Know that itchy feeling when your girl goes talk to another guy, or a guy is hitting on her and you SEE what he's doing? First, if random guys talking to your girl make you nervous to the point where you think she's going to run off with him, you do have some insecurities… and guess what'll happen. It's OK to be THINKING in that situation though, in my eyes. I learned to ENJOY the feeling I get. My brain is trying to tell me something, and it tells me something about myself.