You must have raw wood somewhere embedded within the leaves.
Think of it as glue. The less you have in one area the less things
(leaves) you will have attached to it. Every now and then while you're
building with leaves, just place a wood block somewhere within them.
Leaves have to be within 3 or 4 blocks of regular wood otherwise they will decay. I can't remember which. Once we get 1.8.2, player placed leaves will not decay anymore.
Think of it as glue. The less you have in one area the less things
(leaves) you will have attached to it. Every now and then while you're
building with leaves, just place a wood block somewhere within them.
repeat for how ever long you want to make the bush