Firm the back of the arm while unpacking your groceries.

An easy way to help firm up the triceps muscle is to extend your arm overhead against resistance.  The resistance doesn't have to come from a dumbbell to be effective.  The next time you are unpacking groceries, take a soup can or a 1 liter bottle and extend it overhead toward the ceiling.  Keep your back straight and bend your elbow bringing the can or bottle down between your shoulder blades. Extend the arm straight toward the ceiling again.  Repeat 10-12 times on each side.  If it feels really easy,  grab a heavier can or bottle.