Question: One problem I'm always having is after I get all those pictures layered on top of each other, some of the intensity of the originals pictures tend to get lost. How can I fix that?

a simple way to fix that in Corel (although I'm sure this would work in any program) first I get the look I'm after..... 

1) Then I'll merge it all together. 
2) Then go back and mask of certain areas I want to stand out... 
3) Convert them to a layer... 
4) Then change the look of the layer using the Merge Mode (particularly Add or Hard light) and opacity. 
5) Blend out the edges using the object transparency brush tool, so only the part I wanted stands out. 
6) And sometime repeating this proses over and over, using different filters ....

 this is also quite helpful for giving a collage more depth an stronger lighting. By Erin.

Question: I would like to know how you handle layers. Are you always able to keep your psd file in layers or at a certain point you flatten and then go on working on it again?

Instead of doing that, "copy merge" is used sometimes. "copy merge" is used in order to get a merged image, yet not having to flatten your PSD.

For instance, you've built your collage, now you would like to run a filter on a merged image, not just on each section... up until now you'd flatten the image and lose all your layers... and it's a pain reverting back... but what I do is this,

Make sure you are within a layer that isn't hidden. (Than go to "select" => "select all" or use the shortcut: CTRL+A). Then go to "edit"=>"copy merge". What you did in essence is copy a merged version of the image to the clipboard. now what you have to do is hit "paste" (from the edit menu or the shortcut CTRL+C). Now you pasted the new merged image as a new layer above the rest of the image, or if you were located in a layer in the middle, then the new merged image layer will be pasted above the layer you were in.

Alternatively, instead of pasting a merged version within the exiosting image, just go to "file"=>"new" and open a new image and then hit "paste".

I also would like to recommend working with "layer sets".

What this means is, you would like to run some filter only on some of the layers... what you can do is link the layers. here's how you do it:

In the layer pallette you have next to each layer an "eye icon" (used to view or hide layers), and next to it a little box. If you the layer is currently chosen, you will see a little "paintbrush" icon in that little box next to the "eye icon". Now, to link, you need to click that tiny little box next to all the layers you would like to link with the layer that is currently chosen. You have to mark this box, but you must not choose these other layers. Just click on the tiny box and then the box will be marked with a small "chain" icon.

Ok, once you have linked all the layers you wanted to work on, hit the tiny arrow on the side of the layer pallette and choose "new set from linked" - all the images will now appear under a folder icon which represents a new set. What's nice about a layer set is that you can copy a whole set at once and not have to copy each of the layers one at a time.

I like making a layer set, then duplicating it, then I like merging one of the sets. All the layers in the set are still linked after they are turned into a set. You can then hit the layer pallette menu again and choose "merge linked" - and only the linked layers will be merged.

You can also use "merge down" to merge only the layers below the layer chosen.

You can also hide various layers you don't want to merge and then use "copy merged" only on the non hidden layers, but make sure the chosen layer (the one you are currently working on) is not hidden or "copy merged" won't work. By Vered.