1. I don't want to be embalmed.
What is it? They suck all a corpse's fluids out and inject toxic chemicals into blood vessels, the abdomen, chest, and under the skin; and paint other chemicals and cosmetics onto the skin to hide the fact that the body basically doesn't contain anything natural but skin and bones any more.
Why do it? There's a misperception that embalming kills diseases. But most diseases die within 24 hours once the body's dead; and other diseases (e.g. anthrax) aren't killed by embalming. Other reasons are to keep the body looking and smelling acceptable for the funeral; however, if the body's kept cold enough, it should still be fine for up to four days.
Why not do it? Embalming keeps the body preserved for years longer than natural, as insects won't go near the embalming fluids. When the body does decay, it won't all go at once. First your legs will decay, and your butt. Last — because injected with the most chemicals — are your chest, arms, hands and face, so you end up melting away like some weird decaying mummy from a crappy horror film. And those parts won't stay looking normal before they finally do decay; they shrink.
2. I want to be buried in a shallow grave.
This one's easier to answer. Your body decays in under two years; it's much better for the environment. Deeper graves make the body take longer to decompose, and it's too cold for the soil to properly absorb your body's nutrients. The minimum depth you can be buried in New Zealand is one metre. You can be buried at one metre deep in the Palmy cemetery.
Any other questions?