In all honesty, I gave up on the Slicing tool years ago, I basically just draw my design, then simply choose my images and Copy Merged, then paste and Save for Web... I know this is the long way but in order to get my skins just right, I found that chugging out all the slices was confusing the heck outta me. But to each his / her own.
I spent a couple weeks picking apart the default skins, mainly just viewing them in Dreamweaver with the Expanded mode on in order to see how all the tables were nested. I soon realized that the designs were all very simple table-based designs with a couple tokens thrown in here and there. So now when I'm starting a new skin, I just setup a new folder with xxx_skin.html xxx_skin.css xxx_skin.xml (xxx being a 3letter client abbreviation) and then just throw together a table-based layout like I used to before getting into CSS layouts (quite fun for those of us who have been beating our heads against CSS-pure sites)... I just make sure there's a [MENU], [USER], and [LOGIN] token and I can usually add the rest as I go...
Finally for the content area, I use a cookie-cutter code that I picked up here months ago (note: I also keep all my code snippets in a central folder for easy reference):
<TABLE width="100%">
<TR>
<TD class="toppane" colspan="3" id="TopPane" runat="server" valign="top"></TD>
</TR>
<TR valign="top">
<TD class="leftpane" id="LeftPane" runat="server" valign="top"></TD>
<TD class="contentpane" id="ContentPane" runat="server" valign="top"></TD>
<TD class="rightpane" id="RightPane" runat="server" valign="top"></TD>
</TR>
<TR>
<TD class="bottompane" colspan="3" id="BottomPane" runat="server" valign="top"></TD>
</TR>
</TABLE>
This will give you 5 Content panes, 3 columns in the middle with a "header" and "footer" pane. Incredibally useful. One thing this lil snippet opened my eyes about is the ease of making your own content panes, simply change the class & id names and you can convert any TD element into a content pane that you can drop modules into.
I'm sure this is DNN101 for many but I like to chime in with my $0.02 every now and again.
Slainte!
SB
Edit - Quick note, once the skin is "ready" I ZIP it and upload it thru the skins interface, it will parse it into an ASCX automatically. I still cheat this way b/c I have yet to wrap my head around actual programming. Also, if you leave your XML blank, you will get a minor error when uploading the skin, however in my experience it hasn't broken anything.