What are some ways to prepare beet greens?

It's no accident that, as Ray says, people treat beet greens similar to chard (swiss chard - also known as silverbeet in some regions!). According to On Food and Cooking, they're the same species, Beta vulgaris. Chards are varieties that have been selected for thick, large leaves, subspecies cicla. The same passage also mentions that beets are a distant relative of spinach! Pretty distant, though - same family, different subfamily. Beet root (commonly simply called beets) was on the other hand selected for the root - subspecies vulgaris. (Wikipedia mentions a couple other subspecies, but you're unlikely to have seen them - they're wild, not cultivated.)

As for uses, there's really no end to it - just like there's no end of chard recipes. Unfortunately, since the greens are usually pretty small compared to the beets, it's hard to get a lot of mileage out of them. Since they're generally thinner than chard, they cook down even more than chard does. Sometimes I just quickly saute them and have a snack while I cook the rest of the meal. If you have another dish with greens, you can toss them in there. You can also just cook them separately and toss them back in with your beets, to add a bit of color and texture to that dish.

Finally, the presence of the greens is by no means universal in the US - I've seen plenty of stores selling just the root. I prefer them with the greens, though. Both parts are great, and I enjoy the bit of extra variety in my meals.

The leaves are often referred to as "beet greens", which might return some more search results. I usually see them treated similar to spinach or swiss chard; that is, either served raw in a salad, or blanched and/or sauteed, perhaps with some garlic and olive oil.

I treat the beet as three different items - the root, the stem, and the leaves. I generally use the stems and leaves at the same time, but I cook them differently. Unlike ruby chard, the stems keep their glorious colour when they're cooked, making them something I like to add to lots of dishes for contrast. So in soup, for example, you can saute the stems (along with onions, carrots, and the like) to get them good and cooked, then add the cooked stems into the soup for a simmer. The leaves I cut into half inch ribbons which I stir into the soup at the last minute. Another thing which I do with any stemmed green is to saute the stems and then put in the leaves (usually in ribbons again) with the water that clings to them, put a lid on the saute pan and a minute or two later, serve them.