This topic is for all optimizations you guys know in assembly.. from simple opcode tricks to complicated self-modifying code. These should be more geared to common encounters, so keep that in mind. I don't see another topic like this so I thought it'd be a good idea to start it. I think if each one was presented and given a brief explanation then maybe we could all learn from them. Anyhow, another thing I need to say is that any off-topic posts will most likely be REMOVED.. you have been warned.
Now, to start this off and give an example layout, I'll give the most obvious one that pretty much every Z80 programmer knows (or seriously needs to).
Optimizes for speed AND size (3 clock cycles and 1 byte):
to
Good: Commonly used and is a quick, easy optimization.
Bad: Clears the C and N flags, so if you really need them, this optimization might not work out. Generally it'll be good for ya though.