The Principle of Reciprocity
Give something of value first, before you ask for anything in return.
Humans are wired to repay debts. When someone gives us a gift or does us a favour (even an unsolicited one), we feel a powerful social obligation to give something back.
Buy a colleague a coffee before asking for their help on a project. Provide a potential client with a free, valuable piece of insight before you even begin your pitch.