• Powder: To balance out already applied foundation and moisturizer. No powder brush, just a makeup pad.
  
  
• Compact mirror: Typically with your powder.
  
  
• Eyeliner: If you use eyeliner, blend it with the other side. Always helps to soften the line. Don't re-apply liquid eyeliner unless you are a licensed professional.
  
  
• Blush or shadow: One or the other unless you are going for the painted doll look. Blush is good for nighttime's extra something. Shadow: Use something light so that your eyes pop.
  
  
• Mascara. Use after the eye makeup applications to avoid frosty looking lashes.
  
  
• Lipstick. Naturally! The most essential!
  
  
• Lip balm. If your lips are dry, drink a glass of water (you're dehydrated) and use balm. Wait a sec before laying on the lipstick.
  
  
• Nail file. Always useful.
  
  
• Breath mints or those melt in your mouth things. Just in case.
  
  
• Pony tail elastic. For bad hair days.
  

• There's a time and place for retouching the warpaint. See cosmetic bag etiquette for tips.
• Where to find the perfect cosmetic bag? Try here for starters.
• When is it time to refresh the makeup? Serious recommendations posted by a purser:

Mascara gets the worst bacteria of all the makeup (besides lipstick.) The rule of thumb is to keep mascara no longer than four months with mascara because it is a breeding ground for bacteria. Also, your bacteria doesn't necessarily jive with other people's eye bacteria, so sharing mascara is out. I was once in a play with a gal who got a horrible eye infection from sharing mascara with the rest of the cast.

Here are some expired make-up rules to live by (mostly because makeup sort of expires and gets cakey or gross,) from my friend who is a make-up artist:
*Brushes: Wash every two to three months in a mild detergent
*Sponges: Wash weekly & discard monthly
*Foundation: It depends on the ingredients: A water-based foundation will last up to 12 months, oil-based will last up to 18 months. No need to do this to oil-based foundations.
* Lip liner Up to 3 years.
* Lipstick Some experts say 1-2 years. Others say up to 4 years. What everyone agrees on is that if it smells rancid, throw it out, it's spoiled. Extra tip: If you store lipstick in the refrigerator, it will last longer.
*Mascara: Four months TOPS.
*Concealer: Up to one year.
* Powder: Up to 2 years.
*Eyeshadow: Up to 3 years.
*Cream & gel cleansers: Up to 1 year.
*Eye liner: Ok as long as it's sharp and makes you look good.