Start a "get rid of" pile. In one of our little rooms I have some laundry baskets and bags filled with things I am going to sell in a rummage sale. I add to it as I come across something that needs to go. It will sit there for a while, but I plan on having a rummage sale in late July. If a rummage sale isn't an option, there are many thrift stores you can make donations to. I used to make regular donations (several big bags worth) every six or nine months, until I got a place that had a yard for sales.
An important tip: if you find yourself cooking, or doing whatever and you realize you put something in the sell pile, you can take it out if you're going to use it. BUT when it's time for the sale or donation, you CAN'T go through the pile again. It just has to go. Chances are those things have been sitting there for months without you using them, so you won't miss them. It's just a good way to weed things out.
I also find this tip to be particularly helpful when you have too many clothes and don't know where to start. Go through your closet and look at each piece. If you know it has to go, take it out and add it to your giveaway/sell pile. If you hardly ever wear it or you're not sure, turn the hanger around so it's on the wrong way. After 1 year (or six or nine months, I would think this depends on seasons, too) take those out and give them away or sell them, because you'll know that you haven't worn them in all that time.
I was a bit ruthless a few months ago. I got rid of those "one of a kind" polyester shirts from the thrift store. I got rid of things that didn't fit me. I tailored the things that didn't fit, but were pieces I needed, something that is so easy to put off. Then I made a list of pieces I needed to replace, carried it always in my purse, and only bought those. I'll still buy something that isn't on my list every so often, but I do this way less now. I'm also trying not to buy "throw away" pieces of clothing. Silly things from Target, etc. that are cheap quality and won't last long. I may spend more on a piece, but only if I'm sure it is good quality.
When I got rid of all of that stuff (it started in my closet, but then I went through the kitchen, and storage, and the bookshelf, etc.!) I had some friends come over and take what they wanted. They thought I was crazy but I reassured them with the truth: I still had a closet full of clothes. It's so crazy how much one person can accumulate.
I also try to get rid of one thing if I'm buying another. Like, "I need another pair of sandals, okay these that I hardly ever wear can go because I like my new ones more."
I have the Unclutterer.com blog on my RSS feed. They have lots of great tips all the time.