Right. Crop your image (mine is from ariane179254's lovely caps) and resize. You know this part.



Go to Image > Adjustments > Auto Levels to fix some of that darkness.



Now, I do the next couple of things on pretty much every icon I make, then fit the colouring in between (probably backwards i know, but what the hey) Duplicate the base and set it to Screen, 100%. I normally sharpen this layer rather than the base.



Duplicate the base again, drag it to the top and set it to Soft light, 60%. Use Gaussian Blur with a 1 pixel radius just to smooth it a bit.



Ok. Needs more colour. In between the Screen and Soft Light layers, make a new Selective Colour layer with these settings:

Reds: -100, 0, +100, +28
Yellows: -65, 0, +25, 0
Greens: 0, 0, -47, -79
Magentas: 0, +100, 0, +100
Whites: +6, 0, -26, 0
Neutrals: -18, 0, +11, -22

Leaving you with this:



And just to bump up the colours a bit more, add a new Hue/Saturation layer (still underneath the Soft Light layer) and set the Master saturation at +15.



Finally, add a Colour Burn layer (underneath the Soft Light layer again) in #e2faff (cyan) and set it to 64%.



And thats it. Obviously those values aren't going to work for every image so don't copy it exactly. If you've got any questions feel free to ask :)