Remove everything from the closet. You'll want to be able to see the space clearly to envision how your organizer will fit in the space and best suit your needs (this includes existing rods and shelving). Once you've emptied everything, vacuum and dust all surfaces.
Do some mental organizing; keeping in mind how often you use different items in your closet. Items worn often should be easily accessible. Lesser-used items can be stored on higher shelves or in bins at ground level.
Get rid of unnecessary clutter. If you haven't worn a piece of clothing in the last year, consider donating it. Don't forget to leave plenty of space in your organizer for future clothing purchases and items still in the laundry.