Your bedroom closet is an absolute mess. You don't know what that huge pile at the bottom is actually composed of, half your shoes are missing a pair, hangers and dry cleaning bags are everywhere, and you keep saying, today is the day I'm going to do something about it...well...let today ACTUALLY be the do something about it!
If your closet is full of mismatched hangers, now is the time to go and buy hangers that are all the same in color and size. If you have a tiny closet, consider purchasing or making a closet extender which is a bar that hangs down from your existing one essentially creating another row for clothes to hang on. On the same token, you may want to invest/make some accessory organizers. Lastly, label two separate boxes, "Donate," and "Trash" and set aside.
Take everything out of your closet one item at a time. Create piles for each type of item. These are all shoes, these are all suits, these are all hats, these are all skirts, and so fourth and so on. Put any misc items that are not shoes, clothing, or accessories in a separate pile.
Tackle one pile at a time. Lift each item up and access whether you should keep it (place it back in the pile), donate it, or trash it. Items that should be trashed are ones where they have holes, items are ripped, they are extremely faded, zippers/buttons broken missing, bleach or pit stains, severely discolored, or that are in otherwise very poor shape. Once you have created this box, IMMEDIATELY take it out to the trash and get rid of it so you won't be tempted to let the items sneak back into your wardrobe.
Items to be donated should be in fairly good condition, can be items that still have the tags on them, or items that are far too small or too large for you now. As with the trashed items, bag them/box them up and stick them in your car trunk to go donate or schedule a pick up.
Learn to let go. Step three should not be a long time consuming process. You should be able to pick up an item and quickly let it go, keep it, or decide to trash or donate it. If you have problems letting go or you're focused on the fact that something still has the tag on it despite you NEVER wearing it in three years, have a friend or family member help to let go for you. Someone else who is objective and not attached to any of your clothing items can be the much needed reality check when it comes to deciding if you really need that 8th Hawaiian shirt in your wardrobe!
Now that you have hopefully whittled down your piles, it's time to get organized. If you have a specific work/school wardrobe, place those items in the front of your closet all together including accessories.
Place all off season clothing that you can't wear specifically for weather related reasons right now in a plastic container or vacuum seal bag that you can stick under your bed or in your closet or another storage area until needed. This should help free up some additional space.
Place all long hanging items like long dresses and outfits at the back half of your closet railing system. If these items are not going to be in use on a more frequent basis, place them in garment bags to protect them.
Organize the rest of your closet by both color and type. Use the color wheel: red, orange, yellow, green, blue, indigo, violet/purple followed by black, brown, grey, beige, white, and multi-color. You can do all shirts of the same color, then shorts/skirts/pants, or you can do all shirts one row, all pants/shorts/skirts in another color coordinated.
Help maximize your space by using hanger systems that hook vertically (do it the cheap way by using old coke can tabs to hook hangers one to the other). Use dedicated shoe racks, plastic shoe containers, or hanging shoe shelves to keep those organized. You can use an accessory organizer for belts and scarfs or do it the cheap way using a large hanger with shower curtain loops attached to help string up scarves/belts, and ties.