If you're lucky enough to have an overflowing closet take a minute to appreciate the fact that you haven't had to go without. Many people can't afford the basics so when cleaning out your closet it's a great time to donate. First things first, let's start by taking stock of what you have.
Go through everything with a quick diet and place items in piles based on season and or style. Any items that are ripped or stained and can't be donated should immediately go into a trash pile. You can often still find a use for these items either as rags for the garage/kitchen, filling for pillows or dog beds, cut away the damaged parts and use to see a pillow or quilt. Be creative and see how many ways you can reuse items to avoid tossing them in the trash.
Once you have sort piles start going through each item. This may take several days depending on your available time and commitment level. If you haven't worn something in the past 6 months for in season piles place it into a donation pile. If you haven't worn something in a year for out of season items these can also be placed in the donation pile.
As you view all of your clothing you may find you also have multiple items of similar style and color. Consider reducing the number of duplicate items you have and only keep what you really enjoy wearing. It may feel difficult to get rid of items but you can decide on a length of time to hold your donation clothing (think 5-7 days) and if you really regret adding something then remove it before you donate.
You can and should do this process every six months or so to keep your closet from getting out of control. Make sure to keep the organization of style and season when hanging or folding items so you can easily see what items you have available when you look. Lastly, if you have friends of similar size and taste consider doing a clothing swap on some items you don't truly love any more. This can be a fun way to change up your wardrobe. You can also just see if your friends want to take some of your excess which is beneficial to everyone.