tips for a first time oil painter

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tips for a first time oil painter
i just got into an intermediate figure painting class using oils as a medium. i skipped beginning cause i have painted quite a bit with acrylics, but after the first day i realize they are two very different mediums, and i find myself very discouraged. does anyone have any tips for me? i am thinking maybe there is something major i am missing that is making my painting look so horrible (perhaps it was that summer vacation?)... feel free to reply, here are some of my main questions;

1. stand oil- do you mix it into your paint or just dip your brush in every now and then?
2. do most people mix their colors on a pallete before even laying out the painting?
3. how in the heck does everyone see all those colors in the skin? =) any hints or tips on this (i tried to put in some shadow and it just ended up overtaking the whole painting!)
4. cleaning brushes: straight turpentine? can i let it sit in a jar and reuse the turp from day to day, or is that a bad idea.

thanks so much in advance for your help. (*please email if it doesnt take up too much of your time)