Blackboard writing 101: always break the chalk (to reduce the likelihood of squeaking)
Other points will depend on how much space you have available. Working consistently from left to right helps, although an alternative is working right to left (assuming you are right-handed and have a fair amount of space) to keep your body away from what you've most recently written.
On a whiteboard I use different colours for different blocks of ideas, to provide more of a visual divide. That might not work so well on a blackboard, depending on the chalk available.
Often at undergraduate level there are places where you need to do rough working separately from your main solution. I like to do that on the other side of the board, in a different colour. One of my lecturers would draw the back of an envelope on the board. I guess this is less applicable for 12-14 year olds.
A personal pet-peeve is the smudge marks on a blackboard once it's been erased. I try to be sure to wipe the board using horizontal strokes, so at least the backdrop to the next set of writing is not too chaotic.
You probably need to write bigger than you think you do (although I had one lecturer who wrote way too large). I also took to a completely different style of handwriting to what I use for myself - I think of it as 'baby writing'. Don't be afraid to change your letters if you need to. I've recently started crossing my zs on the board, even though I've never liked that, after confusing one of my own for a 2.