The short answer
Use less than you think, store things away from heat and light, and cut open tubes when they stop dispensing. There is typically a fortnight's worth left in a "finished" tube.
- Cut a tube open with scissors when it stops squeezing out. There is a surprising amount in there.
- Store away from the bathroom window and the radiator. Heat and light degrade actives, particularly vitamin C and retinoids.
- Keep lids on properly. Oxidation is what turns products brown and stops them working.
- Decant travel sizes rather than buying them.
- A pea-sized amount is genuinely enough for most face products, and more does not work better.
What will ruin it
Watch the period-after-opening symbol, the little open jar with a number and an M. Mascara in particular should go after about three months because of the eye infection risk.
Why it works
Most of these are about slowing oxidation and contamination. Air, light, warmth and fingers are the four things that spoil a product, and controlling them costs nothing.
Tipking is general household information, not professional advice. Test anything on a hidden patch first. Full disclaimer.