I realize that it may seem overwhelming, standing in front of your wardrobe and its vast content, but if you go through it step by step, you won't feel as discouraged as you would if you had planned to, say, get it all done in one afternoon.
Seriously, don't overdo it. If you divide your closet into sections, taking on one at a time, the whole procedure is a lot more manageable. You'll start seeing results aka new empty shelf space right away too and won't feel like throwing the towel if you take baby steps.
2. Try it all on.
I know what you're thinking! Why should I put on each and every single garment? Because otherwise there's no way of telling how it fits and suits you (or not). And don't skip shirts and sweaters - you'd be surprised how many you'll actually want to get rid of after seeing your reflection in the mirror.
3. Divide into keepers, maybes and goners.
With most clothes it will be clear right away whether you are going to keep them or toss them out, but there are often those where you're just not sure. Something may fit and look good on you, but will you wear it? Keep those for the time being and only ever throw them out if you haven't worn them by the end of the season.
I recommend the good old closet purge twice a year, usually at the end of each season. By that time you will know what you haven't worn all summer/winter long, which makes it easier to part with those above mentioned "maybes".
5. Keep track of new purchases.
Try not to fill up the space shelf you gained with new stuff! Of course you may always replace things, eg you got rid of some basic white T-shirt and now get a new one, but generally you should first resort to wearing what you already own instead of letting your inner compulsive shopper decide for you!
Food for thought ... I recently read that the average person will purchase 70 pieces of clothing each year. That might seem a lot (and it is) but imagine going on a shopping spree once a month and coming home with 5-6 new garments and you will get this result!
As for myself, I only bought three new shirts this year. I repeat, three. And I only got those to replace a whole stack of old shirts I had already worn out. I'm always happy to set a good example and hopefully I will rub off on you too, my dear readers!
How does my suggestion of a serious closet purge sound to you? Yay or nay?