It’s a small change, but I always forget how to do this so I’m blogging it! Locate the siteMapPath control in the Master Page and add a PathSeperator attribute shown in the example below:
<asp:SiteMapPath ID="siteMapPath" Runat="server" SiteMapProvider="CurrentNavSiteMapProviderNoEncode" RenderCurrentNodeAsLink="false" SkipLinkText="" NodeStyle-CssClass="ms-sitemapdirectional" PathSeparator=" / "/>

