A Good Mirror
Claude Monet was a famous painter who started going blind. He painted bigger and bigger canvases that look like a beautiful wash of perfect form and color from far away, but up close, these masterpieces can look like a mess of spots. Unfortunately, our makeup can look the same; what looks good applied in bad lighting or won't look very masterful up close.
Having a good mirror at home helps you focus in on details from your eyes to your lips, and if necessary, you can be move it from room to room to get the best natural lighting. Keep another small mirror in your purse, car or desk drawer to check your finished face in different lighting and to catch any heavy spots you might have missed at home, like those dreaded clown cheeks or the spidery eyelashes.
A good magnifying mirror can also help with tweezing brows and lining lips and eyes with precision.