After packing up for yet another house move, i really do feel the pain with regard to boxes of "stuff" that is sentimental.
i've had to be ruthless - i used to keep cinema tickets, theatre tickets, plane tickets, entrance tickets... and that was on top of other random keepsakes and sentimental junk. so i binned it. not everything, but the tickets went - i was only keeping them because of the association with memories. i wasn't throwing away the memories, and because they'd been sitting in a box under 8 tonnes of junk, it wasn't as if i was regularly looking at them to remind me of lovely days. same with cards - i used to keep every card S sent me but now, i've just kept a few and stuck them in a book. we've agreed to keep the wedding/engagement cards, but birthday and christmas cards are binned because, heck, they happen every year!
presents from people - if they are in good condition but i still haven't taken them out the box and looked at them in over a year, then they go to the charity shop. if they really do mean something to me, and they are in a condition to be displayed, then i keep them. but i find that there's not a lot like that.
clothes - i was awful with clothes. i've kept three old t-shirts from my yoof but everything else i haven't worn for at least a year went to the charity shop. i used to be really sentimental with clothes and kept just rubbish, ripped and unflattering clothes because of the ticket reason, but again, i wasn't throwing out the memories.
books - this is my downfall. i would keep every book if i could, even the ones i hated. but every time we move, S forces me to charity shop/pass on to friends at least a handful of books.
once i get started, i get into a CULL CULL CULL zone and just start binning things. it's hard to get started but liberating once it's done. plus, the thought of having to unpack or move the junk again in the future sort of spurs me on. 
good luck for the cull - you can do it!