But I find it helps to break things down into tasks that'll take no more than 2-3 hours, and then pick 2-3 tasks I'll do for the day. Once I'm done with those, I'm officially done for the day, though if there're some obvious things I could do quickly, I'll usually want to just do them since I've been so productive. ;-)
You need downtime in order to stay productive. I've found that if I just work constantly and let the days bleed together, I lose my focus and end up getting a lot less done than if I decide what I want to do, do it, and then do whatever I want.
http://www.lifehack.org/articles/productivity/10-tips-for-ra...
and here are some brain exercises to improve concentration:
http://www.egodevelopment.com/10-exercises-for-better-focus-...
Think of it like a swap space for the brain.
And at the end of the day/task, make a note on what to accomplish the next time. This really takes the stress of remembering a lot of things.
My conclusions are that when you go with the flow you end up being atleast as productive, if not more, then when you keep fighting to fit a arbitrary plan/target and you do it without the stress.
Fighting yourself is a completely frivolous activity that can be especially avoided by start-uppers.
I also get up about once an hour or so at work and walk around. When I find I can't concentrate anymore, physically getting up and coming up with some excuse to walk somewhere (the bathroom, the breakroom, the mail drop off) does help me concentrate again when I get back to my desk.