What's the best way to increase productivity?

​1. Don't force yourself to prioritize
Often people make the mistake of putting everything into a priority list and doing the most important thing first, but the first thing is kind of hard to get started with for whatever reason and they end up getting stalled.  Instead, do the most appealing task first.  This will help you gain momentum and once you go, you can more easily get started on the high priority tasks.  This is a processing algorithm similar to "shortest-job first" called "most-delicious-job first."

2. Match tasks to the time of day
Separate your day into A, B, and C times, where A is your highest energy time and C is your lowest-energy time.  For instance, A might be when you first get into work, C might be the dull time right after lunch, and B might be the later afternoon when you get better.  Then separate your tasks into A, B, and C where A is the ones requiring all your attention and ingenuity, and C are the dull mundane tasks you can do in your sleep.  Match up the tasks to the time of day.  Accept that during your C time you are going to be pretty dumb, and don't try to do anything super-great during that time; just do your dull tasks then.  You will naturally rise from your torpor and once you do, you can move on to more important tasks.

3. Consume caffeine carefully  
Treat it as a drug, and make sure you meter out the doses carefully, observing your body's responses.  If you have adverse biological responses to your delivery vehicle (in my case, I only drink sugared colas, and the sugar spike is bad for me), drink a glass or two of water between each dose to smooth out the intake spike.  Water will tend to flush the secondary ingredients like sugar from your body quickly, while the caffeine itself is absorbed into your bloodstream.  Diet sodas and tea don't have this problem.

4. Batch your IMs and emails
It's really not vital to answer your IMs and emails immediately, so turn off your chat client and email client so that you can concentrate on doing work for hours at a time.  If someone really needs you, they will come get you in person or call you on your phone.

5. Stop working when you are really going strong.  
Many people finish up a task for the day before closing up, which makes it harder to get started again the next day.  Instead, force yourself to close up your work right when you are going the strongest.  This will do two things: (1) your subconscious brain will continue working on the task when you're away and (2) when you return the next day, you'll have a bunch of well-defined tasks to get started on, which will help you easily regain your momentum.  This method was a hack used by Ernest Hemingway and apparently works for both writing as well as computer programming.