First, Watch the VIDEO This is a technique I developed to give more of a "wow" effect to photo montages. It's a bit more time consuming, but it's well worth it. To create this eye-catching effect you need to at least be familiar with the basics of Photoshop and After Effects. I'm using Photoshop CS2 and After Effects 7 Pro but any versions will do.
Let's get started!
(Not the full-size version)
First, select a photo that has a good composition with clear layers that you can animate. Look for one that has a clear foreground and background (be sure the photo has enough resolution to fill the screen and enough extra to compensate for motion). I chose this particular photo not necessarily for it's suitability for the final effect, but to show you how to work with different elements (like the transparent veil and out of focus couple).
Get my full-size photo here. (click)

Open your picture in Photoshop. In the layers palette, right click the “background” layer and select “duplicate layer”. Do this again for each layer you will use in your project. I have 3 layers plus the background (which we won't use, the background layer could be used as the background but I prefer to create an additional layer, in case I want to add something). You can also name your layers if you want so you don't get them mixed up. Now, turn off visibility to every layer except the first one.

What we want to do next is cut out everything that won't be in the front layer in our composition. If you're familiar with Photoshop, you know there are several ways to accomplish this. To keep it simple, I just used the eraser tool to erase everything behind the front layer making sure the top layer is highlighted (in this case, the stump with the bouquet will be the only thing left).

Next, select the next layer and turn off visibility to all other layers. In this layer, we will cut out everything behind the couple (we don't need to worry about the bouquet at this time, you will see why later). Before you start erasing, we are going to change some settings first. Because the couple is out of focus, it would not look natural to cut them out with a sharp edge. At the top of the screen, click the arrow next to the brush icon to reveal the size and hardness settings. Bring the hardness setting down low enough so the edge of the eraser tool is about as soft as the edge of the object that's out of focus (the couple). Now when you erase around them, the edge will be soft, just like in the original picture.

One more thing you should try: You may have wondered “what about the transparent veil that has the background shown through it?”. With your eraser tool selected, turn the opacity down to about 40% and erase over the veil (just the part you can see through) in one motion. Now the background layer will be just visible enough to be convincing.
You should now have a finished first and second layer. On to the third.
In the back layer of our project, we need to get a bit creative. If we leave the background as-is, we will have the problem of a duplicate couple showing behind the ones in the second layer when we add motion. So we need to erase them. “Wait...what? Erase them? Is this some sort of sci-fi movie?” Nope. We're going to use a tool in Photoshop called the clone stamp tool. It's the one that looks like...yep, a stamp. Here's how it works. You tell the tool where we want to take a “sample” from; The area we want to duplicate into another area. Then we paint in a new area using the sample we took from another area. Let's try it. With the third layer selected and no other layers visible, hold alt (option on the mac) and click on the tree branches to the left of the bride to take a sample. Now, move the cursor to just over the edge of the bride and paint right over the bride. Now, we don't actually have to paint them out completely, just enough around the edges so that when we put in motion, we don't see their duplicates behind (this will make more sense when we put the image in motion in AE). Also, try not to paint outside the area we are trying to remove, it's already fine. It takes some practice to get the hang of the clone stamp tool so don't be frustrated with it if it takes a few tries.
After the edges are painted out, make all your layers visible and save your image as a Photoshop (.psd) document. Leave Photoshop open in case you need to make adjustments, and open up After Effects. Import the image as a composition. After Effects will automatically make the composition settings match the image, but don't worry about that now (however, I suggest changing the composition settings to about 10 seconds). We will nest this composition into another one later that matches our final output format. Open up the composition which should have the 3 layers separated just like in Photoshop.

Select all 3 layers and hit the “s” key to bring up the scale attribute. With all layers selected, create a keyframe. Now hit the “p” key to bring up the position attribute and create another keyframe. Move and resize the images to the position you want them to start in. Keep in mind that we still need to nest this composition to “crop” the image so be aware of the parts that won't show up in the final composition. Here, I will have the bouquet move from mostly covering the couple, downward. The couple layer will increase in scale, giving the effect that we are moving toward them and the background will also zoom, but not quite as much.

After moving all your layers to their starting positions, move to the end of the timeline (10 seconds) and move them again to where they will end. After Effects will automatically create new keyframes. You can scrub across the timeline to see the effect. When you like what you see, go to “composition” > “Create New Composition”. Create a composition that will match the final format you want (I chose the NTSC DV preset). Now Drag the image composition we already created into this new one. You will need to reposition and resize the composition to fit. Don't create any new keyframes though because the motion is still intact within the original composition.

Once you have the positioning set up the way you like it, you can render the clip. I chose to render as a lossless AVI to be imported into Premiere for the final edit along with the other clips I created.
This effect works best when used SUBTLY. Try not to overdo the effect, it will end up looking amateurish. Also, play around with different moves. Try moving a foreground across the screen slowly and the background in the same direction but faster. Your imagination is the only limit her so go wild and try new things!
I hope you have fun creating “motion picture” montages that your client will be thrilled with!