This is not my own work but something I picked up from a post a came across on MSDN that I thought would be useful, so i thought I'd blog it here so i knew where to find it in the future. .

Credit for this goes to Elizabeth Elizondo.

All you need to do is add this after the PlaceHolderTopNavBar ContentPlaceHolder then tweak it to your requirements.

<asp:ContentPlaceHolder ID="Sublinks" runat="server">
  <SharePoint:AspMenu
      ID="subMenu"
      Runat="server"
      DataSourceID="subSiteMap"
      EnableViewState="false"
      AccessKey="<%$Resources:wss,navigation_accesskey%>"
      Orientation="Horizontal"
        StaticDisplayLevels="1"
        StaticItemFormatString="{0}"
        SkipLinkText=""
        StaticSubMenuIndent="0"
        MaximumDynamicDisplayLevels="0"
        CssClass="ms-topNavContainer">
       <StaticMenuItemStyle CssClass="ms-topnav" ItemSpacing="0px"/>
       <StaticSelectedStyle CssClass="ms-topnavselected" />
       <StaticHoverStyle CssClass="ms-topNavHover" />
      </SharePoint:AspMenu>


      <asp:SiteMapDataSource
        SiteMapProvider="CombinedNavSiteMapProvider"
        ShowStartingNode="false"
        StartFromCurrentNode="false"
        StartingNodeOffset="1"        
        id="subSiteMap"
        runat="server"/>
 </asp:ContentPlaceHolder>

Then you'l l end up with something like this