Sanitize your hands before touching their face. Make sure all your products/tools are clean too. I use the back of my hand as a palette a lot and put products like foundation, creams and glosses there to work off of.
Don't be afraid to pull their eyebrows up to create taut skin or to ask them to look up/down/straight ahead. Use a light hand with little product and build up to the intensity/coverage that you want (this goes for concealer, eyeshadow, foundation and lip stuff too). Keep q-tips and makeup remover close by - the world isn't going to end if you messed up and have to fix a booboo. I find it easiest to do the eyes first, clean up fallout, then go for complexion and lips. Also step back often and take a look at the whole picture of what you're creating.
For wrinkly/crepey lids, stick to mattes or low shimmer. For darker complexions, stay away from too much frosty light colors and go for higher pigmented colors. For asian eye, make good use of highlights to open up the eye area.
Also... TALK to them. Make conversation, explain what you're doing, make them comfortable... From a customer you're selling to and a model who is going to jump out of your chair in 5 minutes, make them love having you work on them!