A few ideas:
Things with sentimental value: take a picture of the item, save the picture, give away/sell the item. Obviously, you're going to keep some of the sentimental items, but you don't need to keep ALL of them.
Things you spent money on: resell them.
Things you might need someday: I like giving things away on Freecycle. Then, if I ever need something, I don't feel guilty about posting a "wanted" item on Freecycle and getting the stuff I need for free. I recently freecycled a ton of stuff when I decluttered my house, and I received a working alarm clock when mine broke and some "new" bathmats.
If you're not sure if you're going to miss some items, put them into a box in an out of the way place for about 6 months. If you need the item during that time, take it out of the box. When 6m (or 3m or whatever cutoff date you're using) goes by, give away or sell whatever's left in the box. I personally like to just give things away- it's less stressful and I can declutter faster.