This is one of the hardest steps of makeup rehab. If you reorganize for the wrong reason, you are throwing away good product in order to buy new...
...One good trick to avoid it is to clarify your goals. Do you want to spend less? Fewer products to sort through in the morning? Be able to move with your stash in the next year? Keep that in goal in mind throughout while minimizing your stash.
Sort your products into loves, expired, and everything else. Loves are those essential products you would rebuy over and over. It's okay to have multiple products in each, but try to be honest with yourself. Throw away any expired makeup, taking note of how much had been used.
At this stage, if you have any family or friends, offer them their pick of your "non loves" pile. I know a lot of makeup "purge" (hate that expression) boards seem to imply this as a last step for your truly awful makeup, but be generous!! If you are not obsessed, and it is a good product a good friend can use, why not??
The other half of this equation, of course, is then not buying NEW stuff to make up for the items that you no longer have. Think of your goal, and how you best achieve goals.
For some people, a no buy is not happening. When faced with an all or nothing, they get worse & start becoming VIB at Sephora!! For others, a no buy helps make a firm decision on any potential purchase ("Should I get this? No!"). The technique that is best will be unique to you, so really consider how you work.