It used to be, even in Victorian times, that women could sport their cleavage generously, but showing off your legs or even your arms was simply scandalous. Life's more liberal today of course, but how can you show off that cleavage and stay classy?
First, it's very important to have a nice, well-fitted, supportive bra. Among other things, having certain "slip ups" will get you a lot of attention, but definitely aren't considered classy. Your bra needs to fit very well so that you don't have a smashed look to your breasts, but rather, a natural accentuation of your curves. You also don't want to show off your back fat by having straps that are too tight. One important thing to keep in mind when going for classy cleavage is the principle of balance. The more you show of your cleavage, the more conservative the rest of your outfit should be. A low, square-cut top draws attention to your breasts, and long sleeves and a long skirt or pants balances the look out and keeps you looking classy. Modesty can be a tricky thing with large breasts, so if you're modest in other places, people are easily fooled.
One of the most powerful secrets to keeping your cleavage classy is imagination. Imagination needs just a little spark and can create a sometimes "overly generous" image of you. When you have people imagining your womanly figure, the immodesty is all on them and you stay perfectly classy. Conversely, if someone can already see you mostly naked, they don't have to imagine what it would be like to get you naked. Show just a little cleavage, and then cover the rest by wearing something that hugs to and shows off your shape. This especially works with larger breasts, since people generally know they're there without you having to show off too much skin.
If you have smaller breasts, try the v-neck tops that are open in the middle, showing off the cleavage area right between your mounds to remind them you're a woman with woman parts. Make sure the rest of your outfit conforms to and complimeants your shape.
Whatever you do, and I can't stress this enough, make sure people can't see your bra straps. In movies where people fly, no one wants to see the cables that are really holding them up. By keeping your bra hidden, you appear more in control of your appearance, and that's vital for class.