1) You don't use it with joy anymore
2) You've found a more contemporary version that performs as well or better
3) You don't use is as often as you used to, or even on special occasions anymore
Notice I said nothing about the expiration date. Beauty products often come with a little open jar logo that has a number inside. It indicates the number of months you have left until supposedly you must buy a new one because the manufacturers say it's the expiration date. Whatever, I still use my Clinique powder eyebrow filler that my mom bought me in '96 and I'm not tossing it.
So what I did toss recently were my Max Factor eyeshadow compacts that contain white, black, purple, beige and grey recently though. I don't think I've touched them for a year, and they had been keeping me from buying replacements that I'll actually use. Near Christmas I bought a new black Urban Decay pot from Sephora for $5, for starters.
Those 3 rules also happen to be my philosophy with clothing, too. When your knit sweater from 4 winters ago is pilling and saggy and it's actually bringing your getup down rather than helping it, then it's time to look at your budget and find a replacement at Forever21 or Gap or Nordstrom.
And you're not a bad spender, you're just being real. So next time you fall in love with a tube of lipstick or an eyeshadow palette and you go, "But I already have that color at home" then ask yourself this: Will you wear your current version as often in the next few months as you would wear the new one that's already stimulating your imagination?