Are you on a 3 task mode?

The human brain has a limit to multitasking. Not going into technicalities but by multitasking I mean doing multiple tasks in a limited time frame (not simultaneously). 

Tasks come in various sizes and shapes. Some require special attention, while as others can be done as a passing thought. In my experience I have found it valuable to pick "3 deserving candidates" for a limited time frame. 30-45 minute time slots work for me but that's very specific to the kind of work I do. I try not to pick more than one complex task in one time slot. Primarily because most of the time, there are a bunch of small tasks hiding behind the complex one. And they pounce on you only when you start working on it :)

When I finish one task - I refill the quota with a new one. If I get something urgent to do - I take one out of the quota. The advantage is that I force myself to focus on only 3-tasks at a time and shut out the rest. Its not as simple I am making (or trying to make) it sound. The key here is to close the file (assume they don't exist) on the rest of the tasks unless they are part of your active task list. It works pretty well for me especially on days where I am distracted for some reason. In the end, it is more important to get stuff done rather than do it all together.