The reason to set priorities is that you can't do everything. That's why good time management also involves deciding what not to do. Too often, we obsess over how to fit every task and project into our schedule, when we would benefit by simply dropping some unnecessary duties.
Just saying "no" means protecting your time from interruptions. For example, if colleague wants to chat, an inessential phone call comes in, or a bill lands on your desk, politely beg off, let voice mail answer, or defer action till later, respectively.
And remember that your most flagrant interrupter may be yourself. You probably don't need to sharpen your pencil, check your e-mail or see what the weather forecast is before you get down to really working. Avoiding procrastination means saying "no" to yourself, too.
Delegating is one way to say no. When you delegate, you're leveraging your efforts by enlisting others. Don't get hung up on the "I-can-do-it-better" syndrome. Maybe you can, but someone else may be able to do what's necessary, and delegating frees up your time for higher priorities. When you delegate a job, let go of as much of it as possible -- don't look over the shoulder of the person who's doing it. Also, consider hiring a virtual assistant to do things like Internet research, or a handyman for a household chore you would waste time on.