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I would like to add a point that I've not seen explicitly mentioned in the other answers. That is that context can often be your aid in clarifying your lettering. As a trivial example, no one is going to misread they as +hey . So the "special alphabet" is primarily for mathematics in your lectures. (That said, you should cultivate a clear hand writing for normal text but it doesn't need to be as extreme as for the mathematical part.)

Similarly, if you never write $\times$ then the students will soon realise that x means $x$ and not $\times$. And I doubt that someone will mistake $x_k$ or $\vec{x}$ for $\times_k$ or $\vec{\times}$.

On the other hand, scale is often hard to see on a board so X and x are not easy to distinguish. It can be awkward to avoid using these together (consider the common desire to have $x \in X$) and distinguishing them visually can be difficult, but then context can help.

Here are some practical tips:

Go in to a large lecture hall, write some random equations on the board, and then go to the back and try to read what you wrote. Then modify your handwriting until you think it is clear. This won't be perfect (since you know what you wrote), but it'll at least be a lot better than not doing it. Say what you write as you write it, and make sure that before you write anything on the board then the students are at the same point as you so that they are paying attention to what you say as you write it. Cultivate writing "the wrong way". It's a bit trickier for those not blessed with sinister tendencies, but if you can learn to write so that you do not stand in front of what you just wrote then it will be much easier for the students to see what you write as you write and say it. This doesn't mean writing with your left hand, it is possible to do this with the right hand but the body position takes a bit of getting used to. This has the added bonus of making you more turned towards the students as you write.

Having just given a lecture, I'd add one more practical point: ensure that the board is clean before you write on it. Stray squiggles left over from before you erased it can be confusing.