Starting from the Eisenhower table, I classify my tasks like this:
It's easy to handle urgent important and unurgent unimportant tasks. The problem is, hardly a task is urgent to me, but most of the time it's just "important", in the sense that working on it will bring me useful knowledge that helps me work on future tasks more efficient. There are many times I wish I has learnt something in the past or has done it before, so that I'm prepared or don't need to do it now. But when the feeling is past or I manage to continue without it, I just let the task slips away.
Since the task is not urgent, it has to wait for the current urgent task. But when the current urgent task is finished, I usually don't know what to do next. All of the tasks in the wait list are important, but none of them is urgent.
My solution is to make a frequency list: every time I see a need to do a task, I increase 1 to the counter for it. When the task or a group of related tasks has high counts, I will work on it. I think this is the way to handle so many unurgent important tasks.
So is there any method that has the similar idea so that I can learn more? I'm also looking for an implement of time management tools, like Gantt chart, but it may be hard, since I can't predict the working time of each task (see also the cone of uncertainty).