We went through this struggle and triumphed, but it was a long road and consistently saying "sit down, sit down, sit down" and then sticking our son back on his bum when he wouldn't listen. We never made a big deal of it on the basis that he could just be seeking attention, so if he didn't sit when we said, we would physically put him down again and ignore any protest. It took a couple of weeks at least, but now he's a champ and never stands.

The key was just persistence and consistency, and staying calm. When I would get frustrated, it helped me to remember that kids test boundaries to make sure the boundary is still there, because boundaries make them feel safe. So I didn't see it as naughtyness (despite the cheeky smile), but as 'seeking reassurance'!