How do you close a coat without buttons?
Plastic snaps (or really any snap that can be applied with a press) are a nice easy alternative to buttons. Just make sure that you use fusible interfacing (on the wrong side of your fabric) to stabilize thin or stretchy fabrics.
How do you close a shirt with a missing button?
If the missing button was toward the top of your shirt or on your pants, grab a sturdy paperclip, hook it through any thread loops left from the button or the surrounding fabric of the shirt, and then slide it through the button hole!