I prefer keeping a list of daily objectives and goals in my day planner, which I can take around with me. Once I finish something, I tick it off. It's really easy to assess whether I was productive or not at the end of the day, and my memory can tell me the reason for any lack of productivity.
A few months ago I got heavily into logging my time, but ironically I started loosing productivity. I create projects holistically, so I'm constantly switching between tasks. Logging how long I spent on a task every time I switched, then analyzing it all at the end of the day just wasn't worth it.
Sticking to my day planner has worked best. In my opinion, time logs help you get into the mindset of being conscious of what you're doing. Once you're in that mindset, you don't really need to log your time anymore, because you know whether you're productive from moment to moment in binary terms.
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