I know I'll show up on time if a friend asks me to help him move on Saturday at 8 in the morning.
I'll work hard to deliver on time when I tell a client that I'll have it done by tomorrow.
If I tell myself that I'll clean my desk tomorrow, I will probably push it to the next day. But my desk will be cleaned on time if I can't help my friend until I clean my desk.
I've been putting my tasks first for 2 years now, and it has helped me get more done. All it takes is for me to move my stuff to the beginning of the day.