I may just be lazy or an incurable optimist (if you knew me, you'd know how ridiculous that last description was
), but my first reaction to a low/flat tire is to pump it up with air and hope/pray that it holds it - at least until I can get home. Of course, if there's a honking big piece of glass or a nail sticking out of it, I might skip that step. But if I can't tell why it's flat, I'll try the minimalist solution first, then escalate as needed.