Early Retirement Extreme is a great resource for some of this. It's like MMM but with more of a philosophical/ecological bent. One of Jacob's recommendations is to categorize things by how often they are used. If something is used on a daily basis, it's probably worth keeping. Monthly, too. Yearly requires some consideration: if you have, say, a giant stand-mixer that takes up a ton of space and only sees action once a year, you have to ask yourself if it is really worth the cost of space you are "paying" to own it. Less than a year are the best candidates for discard. And if you forgot you had something and haven't used it in years, you are probably pretty safe getting rid of it.
As for clothes, the only metric that has ever worked for me is the simple question, "Do I wear this?" I've had tons of shirts I've owned over the years that I thought looked cool, but if they didn't fit right I never wore them. Other times, the shirts fit great but looked terrible, so I never wore them. Ultimately, the only shirts I like are the ones I consistently wear. Once I got this in my head, getting rid of shirts and other clothes became easy. (I do have one or two dress shirts and a suit jacket I only wear for friends' weddings, but I can honestly say I'd rather get rid of these, as their only value is for job interviews and social responsibilities)
