First, this one rule could change your ability to manage your time more than anything else, which is why it's first: If you are a slave to your email system, and particularly if your pc is set up to notify you immediately upon the receipt of any incoming email, then I urge you to make this simple change - it will dramatically improve your control over your time. Turn off the pop-up or noise which notifies you that you have mail. For many people this the single biggest obstacle to successful time management. Establish a new habit of checking your email at certain times in the day, when it is sensible for you and the business to do so - say, first when you arrive at your desk or start work, second just before lunch, third around an hour before normal business closes. You must decide when to look at your emails - this control should not rest with everyone out there who sends emails to you (nor indeed should this control rest with the spamming and virus-spreading community). If your organization has a policy which insists that you be constantly interrupted by your incoming emails try suggesting that the policy is reviewed - involuntary email notification is the single biggest time management detractor in the world today.
Be prepared to make drastic changes. Be creative to find and introduce different ways of doing things. Challenge and question your own habits, routines, and the way you defend your time when others try to dictate how you should use it. The Pareto Principle (80:20 Rule) is a simple easy starting point for assessing where you currently direct your time, and for identifying where your time could better be directed.
Challenge anything that could be wasting time and effort, particularly habitual tasks, meetings and reports where responsibility is inherited or handed down from above. Don't just assume that just because 'we've always done it this way' that it's still appropriate or even required at all. Think about why you are doing things, and whether there is a better way.
Review your activities in terms of your short-term and long-term goals, and prioritise your activities accordingly. Especially, plan preparation and creative thinking time in your diary for the long-term jobs, because they need it. If you don't plan for the preparation you'll never do it, and all the work will get left to the last minute (sounds familiar?......). The short-term urgent tasks will always use up all your time unless you plan to spend it otherwise.
Use a diary, and an activity planner to schedule when to do things, publish or display it, and try to stick to it.
If you are subject to demand and request by others in your organization, and need to recondition their expectations as to your availability and their claim on your time, you should produce a weekly schedule, showing your planned activities and time-slots for everything that you do. This is a vital tool in helping you to explain and justify to others why you must prioritise and schedule demands from others when it suits you, not others.