Okay, I got bored over my lunch break and whipped up a quick set of tutorial pictures. Now, I made this in under an hour, so its pretty rough. Obviously if I spent more time on everything it would look better, but it should give you the idea of how I start out with my clothes.
We'll be making a basic bra/sports top for my most recent model.
1. Block out the basic shape using the mesh you already have.
Obviously it won't be perfect depending on your mesh, but the idea is to just get a rough outline. If you were doing a tight jacket, you'd highlight the entire chest and arms.
2. Copy/paste/recolor
Copy/paste the area you have highlighted as a new object, and make it a very different color so you can see where its crossing through (clipping) the base model. Also, set your base model to Hidden and set your 3D view to Show Hidden. You don't want to mess with the original, but you do want to see how it interacts.
3. Reshape your model.
You don't have to get it perfect right now, but move your vetices around until you get the basic shape you want.
4. Start resizing the edges.
You can't just do a mass enlarge on this, so just go around the edges and start moving the vertices outward so they are no longer inside the base model.