Ever took shots of subjects in dull sky or bad background conditions. Fret not! I have a solution for you to recycle some of your photos in a creative manner. Read on and I will tell you how.

Double exposures in a photographic film term is basically shooting 2 exposure on a same piece of film/slide. If you use a film camera, simply take 2 shots without winding. If you have a DSLR with a multiple exposure setting (eg., Nikon D2x, D300), you could try playing around with it from the menu settings.

For me here, I will simply use Photoshop by using the tools; layer opacity/fill and blurring filters, I can merge the following 2 images into a rather more dramatic output.
1) An Egret taken on a flat colored white sky but somehow I like its flight pose. 
2) At the same time, I turn towards the sun and took this sunsetting shot. It will be nice if its a round yoke but for demonstration sake, this is good enough. 
3) To perfect the background, you need a little bit of visualisation skills and common sense. Here, I try to emulate the shallow depth of field of the 400mm lens as if I was focusing on the Egret as if it was there. In this case, I use the Photoshop’s lens blur function. You could use guassian blur or whichever blurring function you like as ultimately the objective is to make the final merged image believable. 
4) Pull both images into a single file creating 2 layers, one stacked ontop of each other directly. Then, decrease the opacity or fill slider to a desired level where you could see both images are balancely merged. With the Egret’s flat colored background, it actually makes the job easier.
5) Next, merged the 2 images together by highlighting them and pressing Ctrl-E. From here, adjust your curves, levels, saturation, etc. to your desired liking. Now, I did also dodge some lighted parts of the egret off to make the back lighting more believable. So here is my humble attempt.
Now, I believe photography purist will dislike or disagree with what I do but all I can say is this is more of an art and creativity on my part. Even in the good old days of darkroom, one does all the manupilations there anyway. But this is for sure, I do not cut down a tree or disrupt the livelihood of a subject by doing this.

Hope this is helpful and broadens your creativity after clicking the shutter button.
Cheers, Roger Deng