I usually *try* auto, just to see if it's any good.
If it's not I do levels manually.
I used to just use the RGB sliders, pulling them in to the mountains etc etc, holding down the Alt key to check clipping....
Now after reading a CS2 tutorial, I do it differently.
I use the colour sampler tool, I set one sample at what *should* be true black on the picture, one at true white.
Then with the info palette displayed. I show the RGB values of each of the two samples I've placed.
Looking at the values of the black sample, I notice which is the lowest of the three (Red, Green or Blue), I then adjust the others to match it.
I then do the same with the white sample, but instead noticing the highest, and raising the others to match.
It might seem a bit daunting at first, but it usually produces a very pleasing result for me.
the tutorial did use 4 colour samples, the other 2 being a neutral grey and a dominant colour, but I just couldn't get my head round that bit...lol