Morphing skill comes with practice, lots of it too. You have to be someone who is consumed by boobs or asses, or whatever you plan on morphing. The program I found best for morphing is Adobe Photoshop 7. There are many things you have to remember about images: the more real they look, the harder they are to make. The more you put into your work, the more you get out of it. Let's start with the basics.
Shape is an important thing to keep in mind when morphing. Let's take breasts for example, if you wanted to morph breasts, you'd want to make them round–not so round that they are obviously fake. Give them some gravity - make them sag a little.
There are a few ways to make morphs. One of my favorites is the Liquify Tool. With this tool you can expand, compress, and pull on a picture. I've found that this tool is useful in clothed morphs due to the lack of dealing with stretching nipples too much. I use the 'expand' on pictures where the breasts are facing you. I use the 'pull' when breasts are pointed to the side.
After you get done amending your picture, make sure to go over the background with the History Tool. The History Tool will return parts of the picture back to its original state, wherever you paint it. If the edges of your breasts are too sharp, use the Blur Tool. Blur the edges of the breasts slightly to give a more realistic image.
Another method of morphing is the Copy-Expand-Paste method. This is where you expand the original picture. Select the breasts and copy them, then paste it onto the original picture at its original size. You might have to History Tool some of the copied picture off so that it will fit. When finished, add shadow (if needed) and blur (if needed).
There are other methods of morphing that I haven't found out about. Hopefully some kind person will fill this in for me. Good luck, and keep praticing!