There is something I've only just discovered on picnik this week!
It's amazing! you'll just have to play around with it!
It is an advanced effect that allows you to fine tune the color balance.
It is found on the "create" tab under "advanced"
I also like to layer my effects for an even bigger overall result!
so before or after playing around with curves you could make your image black and white, sepia, or do the cross process effect. whatever you want! the options are somewhat limitless...
These are all images created using the curves effect... just dragging the curve and the points around until I liked it. no real strategy.