Compression is the one context technique that costs money to apply, so it is the one that most needs an arithmetic justification. Two cheaper things usually beat it, and knowing when they do not is most of the skill.
The free wins, first
Before any model-based technique, remove the tokens that carry no information. This is unglamorous, lossless, and routinely worth 30–60% on real prompts:
Strip markup. Scraped HTML in a prompt is mostly attributes, navigation and script tags. Extract text.
Minify structured data. Pretty-printed JSON pays for every space. For tabular data, CSV or a compact key-per-column form is dramatically cheaper than an array of objects that repeats every key on every row.
