2. Morph it
This is actually one of the easiest parts of this. In Photoshop 6, duplicate the original layer, and morph the copied layer. In the Liquify window, I usually start with the Bloat tool at a brush pressure of 25 or less. Don't do too much with just this tool; it will make the breasts too spherical.

Then pick the Reconstruct tool, set to Stiff, and run it over the tops and centers of the morphed areas at a pressure of 15 or less. This will help blend the morphing with the original image. "Show Mesh" occasionally to see how distorted it looks; if the gridlines are too far apart, you need to do some Reconstructing blending.

I then mostly use the Warp tool to drag everything where I want it to go. (I think this is "Move" in Goo). Start with a larger brush size to drag outwards, and work down to smaller sizes to take care of necklines, straps, etc. Compare the shape of the morphed breasts to an image of a real large pair to see how they should look. Are they too spherical? Does one stick out more than the other? Does one hang too low?

When you "Show Mesh" on a good morph, the lines themselves should look like a wireframe of a large pair of breasts. Remember to keep going back with the Reconstruct tool to ease the edges back into the original morph. Use Reconstruct set to Revert at pressure of 5 or less around the very edges of the breasts so the edges of the curve drop away properly (the lines gradually get closer and closer together as you approach the edge).