It can be difficult to remember that saying yes to something means saying no to something else. To keep this in mind the next time someone approaches you with an opportunity, ask yourself: "If I had to do this today, would I agree to it?"
Picture: Robert Lowe/Flickr
Think about how you plan to fit into your schedule, what you will probably need to give up or deprioritise, and whether or not it's worth it. Ideally, this will help you immediately feel the eventual annoyance of obligation and prevent you from committing to something you wished you hadn't.
Unfortunately, this question does break down if a task or commitment takes longer than a single day (although you could still pretend that today is Day 1 of a longer commitment and project it throughout the week or month). To help gauge interest, you could also consider whether your reaction is a moderate "Yes" or a resounding "Hell yeah!"
The power of saying no [Tim Harford]