This might not help with the immediate decluttering, but...
With old birthday cards. Its hard not to be sentimental and I used to keep all my DS', then thought 'what on earth is he going to do with 18 years worth of birthday cards when he's 18?' I'm sure he wouldn't thank me for them.
So this year I arranged all the cards for the current birthday and photographed them in a block. Then opened them and took another bulk shot of them all open, so (if you enlarge the photo enough) you can see the messages.
Then, with all digital photos I try to download them all at the end of each month (I usually fail, but sort them out by month after downloading).
So I have a folders for 20111, 2012, 2013 and Jan, Feb, March... within each.
Then, each year I make an A4 photobook (photobox do codes to make it cheaper, but other places will too). So month by month I go through and choose the nicest photos to go into it and we have a nice record of the year (and a gift for doting grandparents!).
Anyway, this year the photos of the cards got added to the pics of DS' birthday party. I put the cards in the art box to cut up... then 6 months later when we hadn't cut them up I put them in the recycling!
I also take pics of any of his major artworks, junk modelling projects etc and put them in the photobooks too, then I can recycle the objects themselves without feeling guilty, as there's a record of them thats more likely to be looked at than if they were stuck in a drawer somewhere getting crushed and manky. :)