I've noticed that I hold onto things for one of two reasons. Either the items were important to me at some point and/or I think I might use them "someday". Both sources of attachment make it hard for me to just throw things into a bag for goodwill, and the stuff just takes up otherwise-useful space. If I'm really attached to something, I have to convince myself that someone else needs or will use the item more than I will. There's really 2 methods I have to force myself to get rid of stuff when I just can't let go.
1. Give them to someone specific
This way, things I once loved don't float anonymously off into the donation bin. I know where they're going and I can pick "good homes" with people I know will enjoy them. (Yes, I'm aware this comes across as a little neurotic).
One example: my collection of Dragonlance books. I was really into the series in middle school and early high school, and I had worked hard to collect about 20 of the books over a few years of used books stores/sales. By college, though, I had mostly stopped reading them, except for briefly introducing a friend to the series at the end of sophomore year. I knew I wasn't going to read them again, but I couldn't bring myself to donate them after working so hard to get my set together.
Solution? Email my friend and ask him if he would take the books off my hands. He gladly accepted the new reading material, and I freed up a shelf.
2. Have someone else tell me to get rid of them
Sometimes I just need another voice telling me that it will be okay. Little sister usually fills this job remarkably well. During a cleaning session last week, I made a small pile of guilt-laden items, and then called her in to ask her what I should do. I wanted some old Winnie the Pooh bags (a duffel and a backpack) out of my closet, so that I could have more storage space. But those bags were my trademarks for a lot of my childhood. I felt really guilty letting them go. So T verbally hit me on the head and told me to donate them. And that was enough.