ALWAYS, keep your hands clean no matter where you are working. If you touch your hair, sanitize. If you touch your face, sanitize. If you touch your cell phone, sanitize.You get the hint, your hands will be in contact with your client's face/body so be considerate of the trust your client has placed in your hands - literally. I always keep hand sanitizer to keep sanitizing my hands in between faces and applications. Sometimes we work in a location that gives us access to clean, running water and other times we are on location outdoors. Always use a steel palette and spatula to scrape product from it's container. You may use your fingers to apply makeup on a client but make sure your hands and nails are clean. Also be considerate of your nail length, which can be uncomfortable for clients and not to mention can be a perfect breeding ground for germs and gunk. I like to have a medium hand sanitizer with a pump in my work station so anyone can use, I keep a smaller one in my pocket or keychain, and I love the EO sanitizing wipes in lavender. I love those wipes because they smell good and they are nice and thick. They can be used to wipe your hands, tables, or chairs.
Makeup Brushes must be sanitized after every use, no exceptions. Once it has touched one clients face it should not touch anyone else's face without being cleaned. You can transfer germs, acne, and other skin conditions. I personally use the Cinema Secrets brush cleaner. I've tried almost all the brush cleaners you can name. The reason I like the CS brush cleaner is that if I have two clients I can clean my brushes in between faces and they will be dry for my second application. The other brush cleaners I have used take too long to dry, especially when I'm working high volume (fashion show, on set, bridal party) I like to clean my brushes as soon as I get home from a job that way if I get called last minute I'm confident that my kit is ready to go.
Sponges & Powder Puffs
One sponge or powder puff per person no exceptions. I like to give powder puffs to my clients so that they can touch up on their own. Most people don't use powder puffs on a day to day basis, so they tend to feel appreciative for the powder puff. It's just a small gesture that clients don't expect. Exception: Beauty Blenders, we all know they cost about $15-$20 per blender. I definitely don't give these away but I do use one blender per person. I sanitize these by running them through a wash cycle in my washing machine at home. I've also let it soak overnight in brush cleaner for hard to remove stains. I follow it up by rinsing with the beauty blenders soap, until all the soap residue is gone.
Pressed Makeup: Eyeshadow, Powders, Blush
Any pressed powders should be scrapped from it's container onto a steel palette. That's the best way to prevent contamination because it doesn't allow anything other than your steel spatula to touch the product. Another accepted way to work and sanitize is to use your clean tools (brushes, applicators etc) to dip directly into the powder. Once you are done with application use a tissue to wipe the top layer off of your product. You can then spray it with 99% alcohol or beauty so clean. Sometimes I use damp a shop towel with alcohol and wipe the top of my pressed powders. Many Makeup Artist use Beauty So Clean which is a cosmetic sanitizer whose main ingredient is alcohol. So that's just personal preference either one works well.
Eye and Lip Pencils
After every use I wipe down my eye and lip pencils with a damp shop towel that has alcohol. I then use a sterilized pencil sharpener to sharpen off new pencil.
Image from Makeup Geek
Everything in your case can be sanitized but it must be clean and organized also. Everyone on set can see your kit, and there is a trust that is created when anyone sees a clean kit. Not to mention it's our job as makeup artist to be professional and sanitary.It gives a bad impression when there is spilled foundation, or powder all over the place.
Image from Centre Street Style
Thanks to Karen Acosta for this Question :)