Kicking the banana bag, best way by far. Also start doing very light shin-to-shin contact in training. Don't ever go full out slamming shins in training, wear the shin gaurds when you want to go hard.
Forget about rolling pins and trees all that other non-sense. As an FYI, the trees thai fighters used to kick (very few still do) were banana trees, which are softer than regular trees. But that's an FYI, don't kick any kind of tree.