I use a very simple technique to maintain a high level of responsiveness: whenever I'm processing my inbox and I decide that I need to reply to a particular message, but it will take too long to do it right now, I file it in a folder named "Reply".
When the dust settles at the end of each day, I have a clean list of the messages to which I need to reply, without other messages clouding my vision.
The "productivity formula" that matches reality pretty well is "Productivity = Knowledge x Focus squared".
This says that focusing hard on something is the most effective way to achieve it. A lower level of knowledge coupled with unwavering commitment can easily outperform someone with better theoretical knowledge who is trying to keep too many plates spinning. You can spend a fortune on tools, techniques and skills, but it is focus that ultimately counts.