During the internship I recently finished, I came to realise how important it is to have a good and structured use of the blackboard when teaching mathematics to 12-14 year pupils. Circumstances forced me to only use the blackboard, a pretty classic chalkboard. My writing on that blackboard can definitely use some improvement. I was hoping for some tips, besides exercise, on how to improve the writing itself and make the total scheme more structured, neat and orderly.
There are probably many tips on this; I remember some discussions of font choice happening here. Let me give only one: take your time.
One easily feels that writing on the board is too slow, and is thus inclined to deform letter, take shortcuts in sentences, and use abbreviations. Most of the time, these are mistakes because it makes the meaning intrinsically more obscure, and it also gives less time for students to understand what is on the board. In most circumstances, teachers should find the writing on board painfuly slow for it to be acceptable for student. Writing full sentences (except at your envelop-back/draft zone as mentioned in Jessica B's answer), taking time to form and space letters will make it easier to have a clean board.
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.
Let students see what you're writing as you're writing. Otherwise, they're bored while you write and must hurry to read when you finish. Instead of facing the board, stand with your side to the board and your chalk hand in plain view, sort of like a "Wheel of Fortune" host. This posture also makes it much easier to talk to your students and not to the chalkboard, which is good policy as well.