1. For ultra-popular attractions (Venice, Maya Bay...), keep jacking up the prices until the tourist flow trickles down to what can be sustained at those places. In case that sounds elitist, have supplemental measures like first-come first-served permits (like the ones at Half Dome, Yosemite) or procedural barriers which only the most committed ones would jump through.
2. Popularize hitherto unknown destinations. I am sure there are tons of gems in "the middle of nowhere". Spread the tourism everywhere around the globe, which will be appreciated by regions looking for economic boosts. Maybe, there are even known techniques like opening up non-stop flights from hotspot tourist sources (Beijing? Shanghai? Delhi?) or filming popular movies at such locations (like The Beach from TFA).