I count three ways to compress data:

1. Make common quantas of data short, uncommon ones long. e.g. Huffman encoding. I, am, not, be, a, or, prestidigitation, gesticulate, onomatopoeia, redundant.
2. Reference known data. e.g. Symbols. ZIP file encoding of references to repeated byte strings. Refer to a whole book’s worth of information by referencing the title. One if by land, two if by sea.
3. Drop information that is not needed. e.g. JPG images. MP3 music. Forget it all. Don’t do it.

Are there any more?

In a sense, all optimization is data compression, is it not?