My mother is a hoarder - it got really bad just after my grandmother died. It was like she didn't want to throw anything away because "Nan would've liked that" or "Nan always liked that". Lately it's got better as she lives in a two-bed maisonette and has realised that she has hardly any living space due to all the books/bags/piles of stuff everywhere. She is definitely a bargain-a-holic; addicted to car boot sales, jumble sales etc. She used to bring me bags of stuff for my dcs and me, things we didn't need, like curtains, etc, and things we would need in about five years time (age 9 jeans for DS who was 4 at the time!)... some things I put in the loft for use later, which is fine within reason, and some things I told her I didn't need so she took them home. I realised that she wasn't getting rid of these things, just hoarding them, so my latest strategy is to take them, say thanks very much, then put them in my loft for a month or two. Then I quietly give them to the charity shop or Ebay them. She doesn't even remember what she's bought me 99% of the time so no feelings are hurt and it's one less thing that doesn't go back into her house!
She doesn't keep broken things, or rubbish, it's just things she thinks might be valuable, or good-quality clothing (even if it will fit nobody she knows and is really old-fashioned). For instance she might buy a Jigsaw skirt, size 16, even though it's not that nice and nobody she knows is a 16. But it's JIGSAW! And it was only £1!!! Crazy. Occasionally she will give me clothes to put on Ebay for her but to be honest I rarely get more than £3 or so for them, so hardly worth it.
Oh, and she's mad on books, even though she rarely finishes one. She will buy hardbacks at bootsales because she thinks they're more valuable 
. Actually she is getting better at sorting through her books and took a load of them to her local church fete :)
My old bedroom is stacked with stuff - bags of clothing, furniture (plus all the stuff I left behind when I left home - by the time I wanted to go through it, it was covered over with junk!). I need to go down there and spend a few days with her going through it all - I bet at least 50% of it could go in a skip.
I find that her hoarding has made me want to NOT be a hoarder. I love books, but I tend to buy from charity shops and once I've read them, out they go. I have a stash of knitting stuff but it's mostly in the loft and I do go through it regularly and Ebay or give away anything i've gone off or won't use. I keep only the bare minimum of paperwork (bank statements scare me!) and I have memory boxes for my kids with one outfit, first shoes, christening cards, bracelet from hospital etc. I am sentimental, but I think the sentiment gets diluted if you have too many of these 'precious' items.
Once I made my mum throw out a Past Times catalogue which was 2 yrs out of date. She got really hot and bothered because "it had a nice picture on the front". I'm happy to say that I think that wouldn't happen now.