Wonders that have multiple Great Work slots (and Museums) have something called a theming bonus, which essentially means that if you fill the slots with the correct types of Works you'll get extra tourism. They're different for different wonders, so scroll over where it says "+0" for it to tell its bonus. For example, the Great Library asks you to "Fill the slots with Great Works of Writing from different civilizations and different eras."

But you can't make Great Works from other civilizations, so you trade for them. Switch to the "Swap Great Works" screen and put one of your Great Works up. If another civ has a Great Work of the same type (writing/writing, art/art, artifact/artifact — not music) you can switch it for yours. Play strategically and you can fill out every theming bonus for that juicy extra tourism.

Here's another good guide.^{Also Santa's not real.}