How do you easily copy a block from the terrain into your inventory in creative mode?

As long as you're in creative mode you can middle-click to copy the block you're currently looking at. You have to be close enough to the block that you could interact with it, and it creates a copy of the block type rather than a true copy (copying a chest doesn't copy the contents of the chest).

You can also do CTRL + Middle click to copy contents of container objects. This includes chests, dispensers, hoppers, command blocks and others.

You can also do it in survival mode, but rather than creating a copy of the block it will just take it from your inventory instead (as long as you're actually carrying that type of block).