World of Warcraft Item Management
Bag Organization
Come up with an organization plan for your bags. The very left bag (or top when you open them all) can be devoted to permanent items that are important. The next bag can be dedicated to holding quest items. The third bag could hold all your trade skill items, and so on. Your default backpack could contain food, water, reagents, and other consumables. It helps to pick different colored bags so you can easily identify which bag your item belongs in. Having a system for organizing your bag space is much better than haphazardly placing any item in any bag, and then having to hunt through each one to find it. Keep in mind you can open all bags at once with the default keybinding Shift + B - this will conveniently let you see all your bag slots with one command.

Bag Types
There are several specialty bag types that are used for specific purposes. They usually have increased bag space at the cost of being able to only store specific types of items. For example, enchanting bags can hold all varieties of enchanting materials, as well as the magic rods used in enchanting. Soul shard bags are large capacity and only hold a warlock's soul shards. Ammo pouches and quivers are technically bags that only hold ammunition, and also provide a slight increase to ranged attack speed with the appropriate weapon type. Using one (or more) of these specialty bags can significantly free up other bag slots for the rest of your items.

The Bank
The bank is an essential part of item management. Store anything that you are not currently using in the bank for maximum benefit. Future upgrades, tradeskill materials, backup equipment, and excess consumables are perfect candidates to store in your bank instead of carrying around. Don't forget that you can get additional storage by buying bag slots and filling them with the largest bags you can find.

Loot
Many items you collect in your travels will be suited well to selling - whether to other players at the Auction House, or to an NPC vendor. Often, the grey items commonly called "vendor trash" are easily overlooked but can yield substantial sums of money when sold to an NPC. It's worthwhile to make sure you always have room in your bags to carry any loot you may find.

Managing Quest Items
Quest items can easily overwhelm your bag space, particularly if you like to keep a full quest log and do multiple quests at once. Keeping your quest items in one bag allows you to easily monitor how many you are carrying around at any given time, instead of having them slowly take over too many slots without you being aware.

Old Equipment
Once you're done using an item, likely because you have found a better one, it's time to retire it. If the item isn't soulbound, you can give it to another player as a hand-me-down or sell it on the Auction House. For soulbound magic items, you can either sell it to a vendor or disenchant it. Other items you may wish to just keep in your bank as a souvenir, and for backup purposes.