Using Unicode (emojis, etc.) may use up multiple characters.
To save characters, omit the closing tags. (technically invalid syntax)
Colorization can be done using HTML color tags: <#FF0000> for red, or can be done using the shorthand form <#FF0> for yellow.
Use \n for newlines (or \v for vertical tabs) rather than <br> for line breaks, to save two characters. Similarly, \t can be used for horizontal tabs, and \r for carriage returns.
Tag attribute quotes can be omitted. Thus <color=red> and <size=9>.
Text is always automatically centered and scaled, thus multiple newlines at the start or end, combined with a fixed size (prevent scaling) would push the text out of the Sign, effectively creating floating neon text!