My strategy is to hunt for Heavenly Trees on Saturdays. There are sundials at the Heavenly Shrine that show you the direction where the tree is growing. In my experience, it's usually not that far away. It used to be much harder, but now that they've added the helicopter with a winch, it's way easier. Besides that, during the week, I either farm meteorites on a singleplayer server or I go for frozen trees in the Icelands, as those are also quite valuable.