I’m a big fan of Derek Sivers’s “‘Hell yeah!’ or ‘No'” rule.
But, even if that gets rid of some so-so commitments, what to do when you’re still “Hell yeah!” about too much stuff?
As Michael Bungay Stanier might say, the trick is to say “Yes” just a little more slowly.
Ask one more followup or clarification question than normal. Ask for a little while to think about it (if it’s too urgent for that, is it really a good opportunity, or just somebody else’s emergency?).
See what happens.