1) Decide whether you like digital or handwritten (PDA, Blackberry vs. Paper)
2) Consider your lifestyle. Do you carry a purse or bag? How big is it? Size and placement in your bag matters.You don't want the planner to get lost and need it to be at easy reach if you have hopes to actually enter dates and record appointments. Test out the size of it in the store by making sure it fits right and establish a "home" for it to be at all times.
3) If using paper, write in pencil. Using pen makes it difficult to make changes and reschedule.
4) A planner that has the times written in it for each day makes it easier to not over book yourself if you can block out specific time that you are doing any one activity.
5) Schedule your to-dos right in your planner, like an appointment, rather than make a to-do list.