Plastic wrap can be used when the skin of a pad is ripped or becomes loose and starts to fall out. Wrap the plastic around the key cup and then tie a knot on top of the key cup.
Scotch tape can be used to reset a cork in place or layers of scotch tape can be used if cork is missing. If it gets messy remove the tape and try again to fill in where the cork should be.
Rubber bands can be used if springs break. Attach a rubber band to the key and fasten it to the horn where it will not interfere with other keys.
If the keys are silver plated or solid silver remove the rubber band right after playing to prevent etching onto the silver.
A small screwdriver can be helpful if a key is binding (sticking): Back its screw off ¼ to ½ turn.
Never use pliers on woodwind instruments. If you notice a bent key, bend it back by hand. Never use pliers to bend a key!