There are probably a lot more than six things you shouldn't say when your new girlfriend meets your ex girlfriend. As a matter of fact, a scenario involving your new girl meeting your ex is one that no guy would wish upon his most arch of enemies. Will all the men in the congregation please say a prayer for any unfortunate brother that has had a run in with his ex and new girlfriend. OK, seriously though, if this does happen to you, there are certain topics that just shouldn't be brought up. Tread carefully.
Anything personal about your new girlfriend. You never want to let your new girlfriend know that you've been talking about her with your ex for a number of reasons. First of all, women can be a little upset at the fact that you're keeping close contact with an ex. Secondly, the ex really has no business in the affairs that you and your new lady share, especially the personal stuff. So don't bring it up. Even if you consider your ex a good friend, you need to face the facts. Your new lady will never fully accept it. So don't add fuel to the fire.
Reminiscing over sexual adventures. Your new lady isn't going to want to take a trip down your sexual memory lane unless she's the one in the passenger seat. Don't even think about bringing it up. Your past sexual exploits with your ex are between you and your ex, and to only be discussed between you and that ex, if at all. Leave them in the past.
Family meetings. Don't talk about how much your family loved your ex. What are you trying to give your new lady a complex or something? You think it's easy trying to win over someone's family? Your new girl doesn't need to know how much your family liked you ex, nor does she need to hear it in a discussion between you and your ex.
Great times, in excess. No excessive bragging about the great fun you and your ex have had. When your new lady meets your ex girlfriend, you should spend more time bragging about how great your new lady is, not how great the past was with your ex. This'll make your lady feel like you're giving her her proper place by your side. To brag about past good times excessively can be perceived as a shot at the lack of good times you're having now.
Don't openly compare the two. There are always going to be things you like about an ex. You may like these certain characteristics slightly more than the some of the one's your new girl has. DO NOT, however, openly compare the pros and cons of each girl while they're standing next to each other. You're not going to do anything but piss both of them off. It isn't so bad if the ex is mad, but your new lady may put you on sex detention for a while. You'll be staying on the couch.
Never, ever, say "I wish things could have been different." You'd have to be a complete moron to even consider whispering this most fatal of phrases. To say something like this totally negates the importance of the relationship you're currently in. It also conveys a message of dissatisfaction with your current woman. You never, ever want to down play your connection with your current girlfriend, be it purposefully or inadvertent.