I only keep cards from DH and DD. If there are nice personally messages in any other cards, I scan them and save them with that months digital photos. I don't throw out the cards, I donate them to St Jude.
http://www.stjudesranch.org/shop/recycled-card-program/
(well, I donate half the card and recycle the other half) I found out about their recycle card program years ago from Dear Abby and they are still doing it.
I have a storage bin that I put all DDs school work in. It fits perfectly on the bookshelf and just the right size for 8.5x11 papers. Once it gets full I go through it and scan the artwork/drawings and anything else I'm proud of and then toss them. There are a few things I saved, like a drawing of our family. Right now, she's only in preschool so I know this will get harder once she starts writing papers, etc.
I'm also in the middle of decluttering. We already had a community yard sale last month and made over $1000, plus took clothes to Once Upon a Child and made another $100. I'm good at hiding the clutter so it doesn't stress me out until I go into our office or guest bedroom (the rooms that normally have the door closed!

) I'm about 75% done with the guest bedroom and then moving on to the office. My biggest problem is that I love to save $$. I know, doesn't sound like a problem but that is what is filled up the guest bedroom. I have enough clothes for DD until she is about 8 (she just turned 4). I pick them up 1/2 price at Goodwill or get brand new clothes for around $2 from Kohls, Kmart, etc. and I just can't resist. I finally have them all organized in space bags and stored in the guest bedroom closet. Along with 2 storage bins of shoes. I've promised myself I'm not buying anymore! We'll see how that goes.... I also buy toys after Christmas on clearance for my DD to give to her friends at birthday parties and have to store them somewhere. So, the guest bedroom will be clean but who ever stays with us will have to live out of their suitcases because the closet will be full. But, hey, at least they will be able to sleep on the bed in a nice clutter free room.
I know most of the stuff in the office is either being filed, scanned or pitched, it's just finding the time to do it. The scanner I bought has definitely paid for itself.