This technique will show you how to blend two images together while maintaining the ability to change the blending without undoing all steps taken afterwords unlike the use of feathering or the eraser.

Step 1
Find two images with roughly the same height or just take them and make them the same height. I will be using the images below.

Step 2
Move them into the same image and overlap them.

Step 3
Go to the layers palette and click the layer for the image on top. Then click the add layer mask button. Your palette should change to the bottom.

Step 4
Choose the gradient tool and make a straight line between the two pictures. Make sure that you have selected the layer mask NOT the actual layer. you do this by clicking the white square next to the layer image.

Step 5
The layer pallete should look like the below picture.

Final Result

This can applied in many ways. I've used it in signatures, banners, and layouts, and there are also other ways to utilize this.