I've been sort of dissapointed with the tutorials I've seen about curves. Yes, you CAN adjust the transfer functions very precisely on each of the channels. Obtaining the very best representation of the tones and colors. But little is said about HOW to do this. I generally adjust white and black point using Levels. Then color balance using the "Variations" dialog. Then curves, but usually just a single center point on the brightness channel. I believe this is more or less equivalent to just using the middle slider in levels. Sometimes I'll get fancy and use an "S" curve, as I think it is often more similar to results in the B&W darkroom. I do not claim to be an expert in this. [I will often use some of Donald's pre-defined curves on a B&W conversion, to mimic the image tones in chemical processes. You can search the threads here about this.] I have noticed that whatever automatic algorithm they use at Jiffy-Mart works REALLY WELL. Digital prints from such places really snap with saturation and tonal range. Difficult to admit, but I wish I knew what they did. It would be a great starting point.