First, watch this: Shin blocks

I would rent Kickboxer and watch how Van Damme kicks the trees, then I would go out in my back yard and try to copy what he does.

Dude I'm joking. Do NOT go around kicking trees unless you live in Thailand and have banana trees.

Do what SevenStar said. A good heavy bag is all you need. If your shins hurt the next day, you went too hard. Conditioning is gradual, if you want to keep your limbs. Consider getting some liniment to put in your shins afterward to speed healing.

Just practice your kicks on a heavy bag, and conditioning will come in time.

Iron

PS. Seriously, don't kick trees.