Summarise for a decision, not for completeness
'Summarise this document' optimises for coverage, so you get a shorter version of everything. Say who is reading and what they must decide — 'a CFO deciding whether to approve next quarter's hiring budget' — and the model can drop nine tenths of the document as irrelevant, which is the actual job.
Say what to ignore
Naming the sections that do not matter is faster than listing the ones that do, and it is more reliable. 'Cover revenue, burn and headcount, ignore marketing commentary' gives the model permission to leave things out, which it will not otherwise take.
Guard against the confident inference
Add: 'where the report lacks data for a claim, say it is missing rather than inferring it'. Summarisation is where models quietly fill gaps, and a fabricated number in a three-bullet summary is far more dangerous than one buried in prose, because nobody goes back to check three bullets.