Embellishments are a more of a problem to store than any other papercrafting supply in my opinion. They come in all shapes and sizes and so many aren't in good storage bags making it difficult to organize into any one form of container. We've talked about eyelets and brads already but I'm talking about everything else now, sparkles, embellishments specific to a paper set, glittery strips and swirls, twist ties, stickers, diecuts, and the list goes on and on.
Let's talk about the new in package items you have. If you've never opened it and have had it for more than a year, you really need to evaluate why. Does it go with a particular paper pack or did you just like the way it looked and so you bought it on a whim? If it's the latter, you probably need to get rid of it. If it goes with a certain paper pack then you need to figure out why you haven't used the paper pack in that amount of time. Letting paper and embellishments pile up year after year is not good for you! It adds guilt to your life. Believe me, I know. When I was not decluttering on a regular basis I would look at everything I had and feel badly that there was so much. That's not fun! It doesn't bring joy to your life, folks! If it makes you feel awful that you're not using some product, get rid of it!
If it's new in package and not over a year old, you may keep it if it goes with a particular paper pack and if you have a SPECIFIC item for which you'll will use it. Be honest with yourself on this one and don't keep what you will "get around to using someday."
Now we get to the very difficult embellishments, the ones that are not new and are in partial packages. I have problems with these myself because they are not usually given to someone else, people don't like used items. I also know that there are times when just one or two of an embellishment is just what I need, especially on cards. So, if you don't have THOUSANDS* of these partial packages you may keep them IF you go through them and evaluate carefully the ones that are worth keeping and not ALL OF THEM is not an answer. Go do 15 minutes.
*If you do have thousands then be ruthless, you will never use them all so get rid of a lot of them. Start with your least favorite and work backwards from there.