FrustratedFrugal - congrats on hitting pan! I will never get there with a blusher, I only have a small number but it is not a product I use often or go out of my way to buy, so the 3-4 I have acquired will probably last me the rest of my life!
Fintan - welcome to the thread!
In terms of getting started, I pulled EVERYTHING out of the cupboards and drawers and spread it all out on the bed. I then used clear make-up bags [had a couple of those 3-part sets from M&S] and sorted into them.
The palettes/kits went into the biggest bag, all my face products [foundations/highlighters/blushers] into another, lipsticks/glosses into one, eye shadows & mascaras into another. I also grouped all shower gels together in a box, and then basic toiletries [deodorants, shampoos, conditioners etc] into another box.
It is really sobering to see that you have 15-20 lippies, many of which are very similar, but it is also really helpful: when it is all there in front of you, it makes it harder to deny how much stuff you have! Plus, with everything grouped together, it is so much easier to find what you are after/pick on a small number of things to use up.
Along the way, as Frustratedfrugal says, you can chuck anything that has obviously gone off or is damaged. If you think you have someone who will like any lightly used products that you aren't fussed on, you can set aside a pile to be donated. I also have a box of things that are new and I know I wont use eg random things from gift sets that don't appeal. If there is anything suitable I give these as small gifts to friends throughout the year if someone is having a bad time.