Minecraft wiki entry for Tree_farming, associated text: Especially if breaking the leaves on the spot, this can be most of a Minecraft day's work. Plan ahead—you may want to wait out the night 10 or 15 blocks up atop the trunk. Every so often, go back down to the ground to collect fallen wood and saplings—if you try to leave them until you've finished, some of the first logs and saplings reach their 5-minute expiration. The fastest way to harvest a giant tree is with a diamond axe and shears or an iron hoe.
If you're using stone axes, bring a spare, because axes get used up quickly. With wood axes, bring at least four of them to be safe. Likewise for your shears or hoe.