Think hard before you put something on the list that you carry around with you.
Whether this is the Reminders list built into Mac OS X and iPhone, or a third-party solution like Clear, Remember The Milk, or Wunderlist, the one thing that these Apps have in common is a lack of motivation to get things done. A feature that they do offer, however, is the ability to postpone or let items linger endlessly. Your only motivator is inside your own brain, and you are the one who controls what gets done in your life. Optimize!
If something is longterm or projected, create a separate “Goals” list for those things. If something is travel related, stick that into a “Travel” list if you can't go there right now, or create separate lists for each place or general area that you'd like to go to.
Try to actually complete some of these lists. Generally, what I've found, is that when you begin getting a lot of things done, you start to more actively compare the completed tasks to the other pending items in your list that you might never do. At this point, it's easier just to remove the undesirable tasks from the list altogether as you move forward to new ones. This is, of course, if you realized much later after adding something that you'd never get around to it. Try to avoid this.
Don't put work-related stuff on your personal task list. This is a big one. Keep your head clear, and keep your lives separate. Even if it's for your own personal projects or business, keep it separate from the things you need to do when you need a break from all of that. (Added 2013/08/25)
Sometimes, you just have to eat humble pie. If there's a task in your list that you don't want to do, but you know you need to do it, spend a day on that task and just get it done if you can. Small progress is still progress, and not every day can be a slam dunk. Alternatively, break that big task up into separate “sub-tasks” that dictate what's left and how you can get this done more gradually. This will allow you to delete the original task that was lingering, and move forward.
For very large tasks (or projects), try to create a separate task list for those entirely and, within it, create small tasks or notes that ultimately “chip away” at it into a much smaller obstacle over time.
Use your calendar. If there's something that you can't do or won't do because it's too soon to be able to complete successfully, don't use space in your task list for it! Create a calendar entry instead that will remind you later in life so that you can forget about it for now. If you end up somehow doing that thing anyway in the meantime, just ignore or delete the calendar entry afterward!
Stay organized and capable. Don't let a massive task list that never cycles become a burden in your life instead of a productivity tool. Keep a clear head, and a clear list. (Is that why it's called “Clear" ?)