Get rid of clothes you haven't worn in over a year.
Take clothes you can't decide about and "put them away" for six months. If you haven't missed them, get rid of them.
Put a small drawer unit in the closet to add more storage space. It can be an inexpensive one, even one of the sturdy cardboard type, since no one sees it.
Store out of season clothes in another location.
Take a rainy afternoon to try on those clothes you can't seem to get rid of. Once you see them on, you'll be better able to decide about them.
Try to get over the sentimental attachment to things you'll never use. Your Aunt Mary isn't going to know you threw out the shirt she gave you ten years ago that you've never worn.

Take clothes out of dry cleaner bags. Changes in humidity can cause yellowing.
Put a light in the closet and leave it on for short periods of time in humid weather. Leaving it on too long can cause discoloration. Use a low-wattage bulb (40 or 60 watts.)

Let clothes air out before re-hanging. Moisture from your body gets trapped in them when they get back in the closet and may cause mold growth.
Don't let cedar chips touch your clothes as they may cause yellowing.
Invest in good clothes hangers. Wire hangers leave marks on the clothes and don't give good support.
Keep your shoes well-polished. They look better and last longer. Once every few months take them to have them professionally polished.
Rotate shoes if possible. Shoes worn every day wear out much quicker.
Let clothes air out before re-hanging them in the closet. This will keep them clean and fresh longer and save on dry-cleaning bills.