If your WAN is truly undersized -- and you can’t afford a long-haul frame-relay network -- traffic shaping and caching can help unclog the pipe.
Traffic-shaping configurations are more art than science. Prioritizing apps is often more political than technical but may have tremendous effects on perceived network performance.
Caching is a different beast altogether. It requires less work than traffic shaping, but the impact will likely be smaller. Caching engines store and serve up local copies of commonly accessed data to reduce WAN traffic. The downside is that dynamic content can’t truly be cached, so e-mail won’t enjoy the same performance bump.