The easiest situation is that you've simply allowed things to get out of hand. You have too many DVDs you don't watch, clothes you don't fit anymore, or paperwork you don't need to retain. You'd rather have the space than their company, and you don't have an emotional attachment. In this case, I would always recommend piles (or boxes, but it can be demoralising if you don't have enough boxes, and you might stop). Make one pile for the charity shop, one pile for the recycling centre, and one for landfill. I have seen recommendations for another 'not sure' pile. If you have stuff you're not sure about, I would actually put it back on the shelf - this isn't a one-time only thing, and next time you go through your stuff it might be easier to let go of it. Be ruthless, and don't stop to look through old pictures, or reminisce. If there is stuff you need to go through in detail, put it back on the shelf - that's to do in between now and the next time. The only other thing I'd have is a shredder, and some cleaning equipment, for when you clear out an entire cupboard, corner or room.
The next scenario is downsizing - getting rid of other people's stuff (usually kids'). The one watchword here is warning - never throw away someone else's possessions without warning them, preferably getting their permission, and crucially, ascertaining the item's worth. Everyone has heard a horror story about trashed mint condition Star Wars figures or priceless comic book collection. If you're selling the family home and there's a deadline for removing items you can't take to the new house, then this can be difficult, especially when the owners don't have storage or maybe aren't in the country. However, if they are adults, then the stuff is their responsibility, and sufficient warning should be accompanied by a clear message, and a determination to bin what isn't claimed. What you decide to keep of your precious memories of when they were babies, is slightly more difficult. Aim to keep only what really resonates with you, and always aim to condense what you keep. Consider a framed picture montage of their earliest artwork, for example, rather than the originals (which will degrade and spoil - especially if they feature stuck on pasta).
One of the most difficult scenarios is after a bereavement. The compulsion to keep everything, and store it to sort out later, perhaps when you're feeling stronger, is a very powerful one. It is not always helpful, however, and is likely to prolong the pain, not only for yourself but also for other loved ones, who might want to move on more swiftly. For this scenario, I would recommend enlisting the help of a more distant family member, friend, or possibly a professional. There are many services offered online to help with decluttering, and having someone who isn't part of the bereavement can be a steadying and motivational presence at a difficult time. In these cases, it is paramount to sort out everything that might feature in the will, and also to identify mementoes for individuals, or help them choose their own. The key here is to be realistic, rather than ruthless, and to try to keep strong emotions, which might prompt you to attach meaning to inappropriate objects, in check.
There are, of course, lots of different reasons why someone might be decluttering, and I've only touched on a few here. However, I hope I've highlighted some of the differences in these situations, and some of the different approaches you might take. I hope that this post helps you decide how to approach your decluttering, if you were unsure.
Thanks for reading. Next time - who knows - I haven't decided yet :)