Do What You Want To Do

This may sound a little backwards and out of line, however in most cases you should do what you want to do at the time.

You should have main goals and work towards those, but of those goals there are usually lots of smaller tasks that all add up to the whole. Of these smaller tasks you should do the ones you want to do at that time and work on other ones at a later time. Maybe you're in a more creative mood, do the creative task. Maybe you're not in a creative mood, do the rote task. Maybe you're burned out. Do tasks from another goal of yours.

I think the key to being productive is doing the things that you want to do at that particular time, so long as it fits into a larger goal of yours.