PERSONAL MEMORIES: I have a file in my file cabinet for my own personal memories of things I definitely want to keep (cards from my kids or husband, etc.)
GENEALOGY-related: I put funeral remembrance cards in my genealogical box if they are a relative. If they are not a relative, I sometimes keep them in my personal memories for a time...but you can't keep stuff like this forever, I do know that, but it's so hard.
WEDDING CARDS FROM OUR WEDDING: I kept all our cards from our wedding for over 10 years and then threw them away and I FOREVER have regretted that. I wish I had put them in a scrapbook.
BABY CARDS: My mother put all the cards they received at my birth in my baby scrapbook and I love looking at those cards occasionally and remembering people. I saved all my kids cards in their own scrapbooks until they got to be about 10 or 12..., thinking they may also enjoy this someday.
PICTURES - NON-IMMEDIATE FAMILY: For pictures of non-immediate family, I usually keep those, but they are in a scrapbook/album (and I am NOT a scrapbooking person---I'm just talking about a plain scrapbook- pictures adhesed and with names written) My sister, for instance, continued to give us 5x7 of her kids' school pictures as they were growing up...I never had those on my walls although she said "these are for your walls" (I have enough trouble keeping current pictures of my own kids displayed!) But I kept the pictures and some are in that generic scrapbook/album of "other pictures." (you'll find you get a lot of pictures of high school graduates, too.) I mostly save those, put the name on the back because believe me--you will NOT remember all those names in later years. I no longer save graduation announcements or wedding announcement/invitations unless it's spectacularly beautiful and then I only have it out for awhile and then it goes in my personal memories file which I occasionally purge the extraneous stuff.
KIDS SCHOOL STUFF: For kids school papers, I used to keep every paper, drawing, test, etc., in a box (a paper crate box works well) until the end of the school year, and then just glean a few of the best things, most all the artwork, special projects, or anything the child is particularly proud of, and then transfer that school year's stuff to another box for that kids. All my kids are grown, but I have a box of "memories" for each of them, and these school papers are included...also things like 4-H ribbons, cub scout badges, drill team certificates, academic awards documents, etc. They can have them someday, if they won't just throw them away...I don't mind the space on the shelf in my house for now--someday they'll have to go through it, LOL!
CHRISTMAS CARDS- I keep Christmas cards for one year. I do look through them several times throughout the year. (I have heard of families keeping these on the table and looking at one every evening and praying for the sender throughout the year--sounds nice--but it was never done by us..) When it's time (like now--November) that you start getting new cards, I go through it one last time and maybe reread the letters and notes, and pull out any pictures I want to put in my "other pictures" collection and then throw them away unless it is spectacularly beautiful....I save maybe a total of one or two a year.
OLD STUFF: I still have birthday cards that my grandma sent me over 40 years ago...I love seeing her handwriting. Right at the time I started getting interested in genealogy, my father-in-law told me he had just thrown away his parents marriage certificate! I asked him why in the world he did that and he had no answer. They had both been dead over 40 years at that point- I couldn't believe it! So think about it before you throw away something that may really be of neat value like that...
HOWEVER- I am a pack rat and I have way too much junk in my house and perhaps you really don't have the space and maybe it is wrong of me to place this much importance on STUFF... so if I am encouraging/pushing you in an area where you have "problems" --I apologize.