apart from the obvious! 
ds, 15 months will, if I don't stop him, step straight off a step at the top of the stairs, same with the slide etc. I stop him, get him to sit down, tap his bottom and say sit, his sister shows him how she sits at the top of the slide/stairs but it still hasn't sunk in. It's more like he likes launching himself to see what happens which would be fine if it weren't in dangerous places.
he wants to climb everything but then either can't get down or will hurt himself badly someday
if he'd only been doing this for a few weeks I could understand but 3 months later and he still wants to launch rather than sit.
or is it just a boy thing?