Effects in Gimp
First start with a photo that has contrasts of light and shadow. It works well for indoor party shots. Always duplicate your base. This means you are working on a copy and need not worry about damaging the original.
Go to colours:Levels. In this case pressing auto worked best. I liked the way the colours looked but you can change it manually using the dropper tools.
I then went to Curves and made a very slight S Curve on each channel:
I then went to curves again to up the contrast a little like this:
Flatten the image then sharpen using the Unsharp Mask. Then crop, keeping the aspect ratio fixed and in Portrait mode. The rule of thirds grid really helps when you are deciding where to crop.
And you're done!
Other example using this technique:
