Postshading

No, i just make sure what ever i am useing to shade is darker than the paint it is going on.

For me, it depends on what the base coat was.  I usually go with tans over greens or browns, light greys over blues, the only time I use white is over grey.  I usually just mix as I use it, but I am using Vallejo eye dropper bottles, so it's easy to count my ratios.  A thin blending layer  (3/4 thinner) often helps as a last coat for me.

I only mix up a little more than I think I will use. If you don't mix enough, you can always mix more later, and you do not need to make an identical mix, a little variation is fine.

I typically mix what I'm looking for in the cup of my airbrush. I use a Paasche Talon and it has a very large cup so I rarely run out of paint and I've learned about how much I need to do specific tasks. I have switched to painting primarily with Ammo paints that come in the little dropper bottles and they require very little thinner, if any. And, like Don stated, a little variation in color doesnt hurt if you do need to mix more, I actually think it makes it look more realistic as it gives some tonal variation to the color if it isnt the same across the surface of the entire kit.