By using your words. If there was a way you didn't like to be kissed, presumably you would speak up rather than endure lousy kisses. Same applies here: "I have big boobs, and they're great, and I love them—but 'YAY BOOOOBS!' makes me feel like I'm only my tits, which isn't a nice feeling. That said, I don't want my boobs ignored, either. The sweet spot really isn't that hard to hit—enjoy my boobs like you would any other nice body part." That said, some people really, really like big boobs and it's going to be hard for them to contain their excitement. "YAY BOOOOBS" could be an understandable and forgivable first reaction on their part and an opening that allows you to have a conversation about bodies, consideration, and consent.