Cutting down your collection & decks

[Sage_Stinson]

I think the question should be, do you have enough space?   

[Judaspriester]

I almost don't want to randomly donate them because some cards do gain value out of nowhere.

I somewhat agree with you, but on the other hand.. will there be enough cards that gain value "out of nowhere", so that it will be worth keeping them?
If I would know someone that would really play with my junk boxes, I would give away like 3/4 of it for free. I might give away some cards that could spike in price, but hey, better the cards see play than they just catch dust.

[Xaarvaxus]

Very true. I always tell my friends that it's only junk if you can't use it. I try to focus on using the rares in my binders and then uncommons that are necessary for the decks. I've got gallon sized ziplock bags of basic lands and non-rare cards in my 800-count boxes. I almost don't want to randomly donate them because some cards do gain value out of nowhere.

While there will always be cards that are initially overlooked at first or don't prove useful until something complimentary gets printed, it's still relatively easy to makes some huge cuts to your collection.  Yoked Ox is very, very, very unlikely to ever become valuable.  I keep exactly one copy of a lot of cards because another tribal bear with first strike isn't going to make my deck, same thing with yet another reprint of Divination.  At most, I may have 4 cards in a set that I keep extra copies of because I think they may have some potential to creep up.  There are some that are kind of obvious, like Mystic Sanctuary.  While I don't think it will ever be $20, a couple extra copies so I don't end up paying $2.50 in 5 years to get back something I gave away for free is worth the extra space.  Most deal X damage red instants and sorceries are interchangeable, Abrade was worth keeping some extra copies of [though more for non-EDH].  The hard ones to evaluate are the cards that do something totally different but don't seem to have much use, at least not yet. 

[stuffnsuch]

I've got a nice large 5 row box with each color, and I've decided that I'll never let any color fill more than one row.  I've got a smaller box for artifacts and multicolored cards.  I've kept some cards because of sentimentality or market value, others because I find them interesting or unusual, some because I just think they would be good if I ever had the right shell for them, so it was mostly duplicates and vanilla creatures and spells that are essentially the same each set (how many blue enchantments do I need that prevent the enchanted creature from untapping?) that I ended up giving to students in my after school MTG club.