This is admittedly a completely unproven theory, but I firmly believe that the single greatest inhibitor to productivity when we are away from our desks is the absence of a writing implement. It´s one of those Murphy´s Law things — you will need a pen precisely when you don´t have one. How often have you gone looking for something to write down an idea or an address or a number or a musing with only to be completely unable to find one anywhere. On the other hand, when you don´t need or want a pen you are almost certainly in danger of drowning in a sea of them. I will leave it to you to determine what forces control this world, but whatever they are they have a sense of humor.
With that in mind, one of the simplest and easy to implement boosts to your own productivity is to make sure that you always have a pen with you. Seriously. It´s not just that you will have one when you need one, but you will avoid the frantic scramble to find one and the ensuing frustration that that causes. Lingering frustration is a productivity killer.
I´m not even going to go so far as to suggest you need something to write on, either. Baby steps. You can always find a scrap of paper, or a hand in a pinch. Just start with a pen. If you aren´t in the habit of carrying one with you wherever you go, you will be quite sincerely shocked by how useful it is to carry one.
Don´t go cheap, either. I´m not suggesting that only a solid gold pen that costs more than my car is the only possible solution, but you want to stay away from anything that comes in boxes of a dozen or more or costs less than 50 cents. That´s not just being a snob, either — there is nothing less productive than an exploded pen in your pocket. Besides, if you have a pen that you like the feel of or look of then you are far less likely to misplace it and far more likely to remember to get it back if you lend it to someone.
There are the pencil faithful, too, but don´t fall for their hype. They say that pencils are the answer because they can write any time and they don´t run out of ink. Sure, but they break, they are hard to read if you stick a pencil-written note in your pocket for a while and it is both difficult and potentially fatal to scribble on your own skin with a pencil.
The most important thing, of course, is where to carry the pen. Go for the pocket of your pants. This is not up for discussion. A bag is not always right near you, a wallet is just inviting danger and a shirt pocket will, quite correctly, open you up to mockery.
Once you start carrying a pen you´ll never be able to give it up. It´s like smoking, only good for you.