Your working life is shaped by the requests you decline as well as the ones you accept. Give those choices a clear basis before a persuasive opportunity arrives.
List the conditions a project needs to be worthwhile: an understandable purpose, suitable authority, workable scope and enough capacity to deliver. Add conditions that protect existing commitments. The list should reflect your actual circumstances rather than an idealised business persona.
Apply the conditions fairly
In a fictional enquiry, a buyer asks for a complete reporting system by tomorrow without providing source material. Clarifying a smaller useful deliverable may reveal a workable project. If the essential conditions remain unavailable, a respectful decline gives both parties more reliable information than an enthusiastic promise.
Explain the relevant limit without judging the person. Where appropriate, suggest a different scope or an alternative source of help. Keep private assessment notes proportionate and factual.
Review declined work occasionally. A repeated pattern may reveal a useful offer you could develop later, or a boundary worth protecting. Choosing deliberately lets the next suitable opportunity receive attention that would otherwise be consumed repairing an unsuitable agreement.