The dress will need to have the same center as the character when you import it back into Poser (I think). In object mode, select the character,then press Shift-S for the snap menu and choose Cursor -> Selection. This will snap the cursor onto the center point of the char. Now select the dress and click the center cursor button(in the mesh panel (F9)) This will snap the center point in the EXACT same place as the character's center point.

Creating and using a lattice is pretty easy now. First add the lattice, than scale and position it to surround the area you want to affect.(making it ever so slightly smaller than for the desirable effect). Next, in the Lattice panel, you can break it up however you like by increasing the U: , V:, and W: variables. To associate with your mesh, just select the dress and add a lattice modifier in the stack. In the Ob: feild, enter the word "Lattice" (the default name) No parenting or anything required.

To make the lattice only affect a portion of the dress, you must create a vertex group. Highlight the vertices you want the lattice to deform. In the Links and Materials panel, under vertex groups, click New, to create a new group container,then click Assign, to fill the new group with the selected verts. Name the group "somthing". In the modifier stack, find the lattice modifier you created. Under the Ob: feild you will see a VGroup feild. Here you will enter the name of your group ("somthing"). Now only this section of the mesh will be affected by this lattice. Remember, you can have as many different groups as you want, and they can all share any other like vertices, so don't worry about all your different groups getting mixed up. (I realise you will have several different groups for your dress as you progress through your work)

You have probably already learned all this info, you seem to be catching on to everything pretty quick :)

Personally I prefer just using the proportional edit tool (toggle on and off by hitting the O key). You can grab a vert and it will move surrounding verts also, how many and how much depending on the strength (mouse wheel) and the type of fall-off you have chosen. Using this along with the 3d transform manipulator (with orientation set to normal) you can do some pretty easy mesh manipulation.

I like your tutorials. Nice style and approach :)