Oh, well, if we're going to be on topic:
OP, buy one of these: Amazon.com: CENTIMETER GRID DRY ERASE BOARDS: Toys & Games
Make sure all your kanji take up a 2x2 square. Use the crosshairs to make sure your characters have the right proportions. Then just practice, practice, practice.
For English penmanship, they tell you you should start by writing bigger than you need, and try to use the elbow and shoulder rather than the wrist and fingers. Writing small in English is supposed to come after you've mastered writing large. People who write with only their wrist and fingers generally end up with cramps after long periods of writing, and if you ask them to go to the blackboard, they just don't know how to write large letters. I don't know why it would be different for Kanji.
I don't know how the natives are going to react to this one, but when I saw this I didn't stop laughing for a long time:
Originally Posted by Nyororin
Whether you are able to read it or not is not the point.
I agree with your point 100%. It made me laugh because, as soon as I figured out it was vertical English, I immediately started at the top right. It supports your point, as the only vertical reading I do is Japanese. I actually forced my way through the whole thing, right to left (which took just as long as you would expect), then stared at it for a painful 30 seconds before realizing why it didn't make sense. And then I just could not read it left to right. I had to read it right to left again, and then reorder it in my head.
Also, since it might interest you, the jumbled word trick starts to fail when you break up the contour of a word. They've done eye motion studies that show that ascenders and descenders (bdfhijklt and gjpqy) are key to reading quickly. Break up a word's ascender/descender pattern and suddenly it's not so phenomenal.
Originally Posted by KyleGoetz
Aha! A little googling around reveals there's a property in CSS3 (the new standard currently under development for document appearance on the web) that enables 縦書き.
Also, I wish English had words as short as 縦 and 横.