Our application is currently using tabs for its sub-navigation. The problem is, on some pages, there are more than 6 tabs; some of its labels are long, so we have had to shorten them.
One idea I have is to switch to using (left) sidebar navigation. The issue with this is some pages can be quite wide where horizontal real estate is needed - particularly pages with dashboard graphs and tables with several columns (in some pages, horizontal scrolling had to be implemented for tables with several columns).
I'm thinking of allowing the sidebar to be collapsible, but I don't think this addresses the core issue, because in cases where the user doesn't collapse the sidebar, we're still cramped for space.