Instead of letting tasks you're not quite committed to loiter on your to-do list until you're sick of looking at them, move them off to a separate list, a holding area for Someday/Maybe items. 

Only concrete actions you're committed to completing should live on your to-do list.

Your to-do list is not your project list. Don't add multi-action tasks to your to-do list, such as "Clean out the office." Break projects down to smaller, easier-to-tackle subtasks.

The smaller and more atomic these subtasks are, the more doable they are. Break down tasks into five-minute increments.

When you have a multi-action task, keep only its next sequential action on your to-do list. When the task is complete, refer to your project list and add its next action to your to-do list.
At any given moment, your to-do list should contain only the next logical action for all your working projects. That's it - just one bite-sized step in each undertaking.

When you tell yourself to do something, make it an order. 

An item such as "Acme account checkup" doesn't tell you what has to be done. Make your to-do's specific actions, such as "Phone Rob at Acme re: Q2 sales." 

When formulating a to-do, the onus is on your Boss self to make it as easy as possible for your Assistant self to get the job done. 

Arm your Assistant self with all the details she needs to get your work done. 

For example, if you have to make a phone call, include the name or number.

Just as no one wants to look at an email inbox with 2,386 messages in it, no one wants to have an endless to-do list. It's overwhelming and depressing. 

Instead, keep your to-do list under 20 items. Your to-do list should be short, to-the-point commitments that involve no more deciding as to whether you're actually serious about doing them.

Although your to-do list might have 20 items on it, the reality is that you're going to get only a couple done per day (assuming that you're not writing down things like "get up, shower, make coffee, go to work...." 

Make sure the most important tasks are at the very top of your list. 

Although my to-do list is only 20 items or so, it's 20 items that change every day. Every day, two to five tasks get checked off, and two to five tasks get added. 

Your to-do list is a working document.

Schedule a 20-minute meeting with yourself every Friday or Monday to review your to-do list, project list, and someday/maybe list.
Use that time to rewrite any items that aren't broken down as much as they should be, purge irrelevant items, and move the next actions from your project list to your to-do list.

Your "done" list is a great indicator of whether your to-do list is working. If more than two days go by without a new done item, it's time to revamp your to-do list and get back to best practices.

90% of the work involved when you're tackling tasks that matter is the planning. 

The more you practice the art of creating effective to-do's, the faster and easier it will come to you, and the more you cross items off your list and leave the office with that delicious sense of completion.