Help with blend modes (color burn/dodge)? (PS7+)

[First post, please be gentle!]

I've seen a few comments lately in discussions of icon-making along the lines of "of course, everyone knows that a color burn layer makes everything more vibrant" (or it may have been color dodge; I can't remember now), but I've never seen any actual reference to this technique in a tutorial.

For those of you that use these layers to make an icon more vibrant, can you help?

* Is it color burn or color dodge?
* Is it a single-color fill layer, or a duplicated layer of the main icon image?
* What kinda opacity? (or is that an experiment-by-image thing?)
* Any other general input about color burn and color dodge blend-mode layers?

Thanks for any input you can provide!

[I'm using Photoshop CS and Photoshop CS3 Beta, but this is probably relevant for most versions of PS, and maybe PSP too?]